Disclaimer: all written below is just my opinion. I tried to be as objective as possible.
(Viki is full of reviews on this drama i'm surprised there are not many in here. and surprised it doesn't have a high rating)

Let me start by saying that I loved this drama!! It is about family, justice, righteousness! I don't usually watch long episode dramas, however I started it without knowing that it will be a 50 ep drama, but I was so hooked after the first ep, that I couldn't stop. Watching it every week was like a torture lol so I put it on hold and then picked it up again. I didn't give it a 10 for some reasons, I will talk about them below.

I want to start with the words Dol Mok said by the end of the drama: "The law now is like a very huge net that lets go of all the big fish that lives in a castle, and it becomes a very tight net that doesn't let go even of the smallest fish that is outside of the castle. The society where law protects only the ones that live in a castle and makes others crumble is a rotten society, and won't last long. So I want it to change with this judgement today."

Story: I think this is the strongest point of this drama! The plot summary doesn't say anything about it actually. This is a story about the descendants of Korean Heroic troops a little squad of about 5 people, lead by General Baek San, that fought for independence during the Japanese occupation of Korea. And the Pro_Japanese family that actually betrayed Korea. It's about the unfairness, how the traitors back then, live a luxurious life for 3 generations, while the heroes that brought freedom to Korea live in poverty for 3 generations. However the 4th generation came to change the tables and to bring justice to the world! The title - good thief, bad thief sounds in Korean like "doduk nom, doduk nim", which can be also translated as punk thief and Mr. Thief. The so called Mr. Thieves are the powerful rich people that steal money from poor people, evade taxes, embezzle money etc. and the punk thief is our main hero who is like a modern Robin Hood, stealing from bad giving to poor (not exactly but something like this). Going back to the story: It had so much twists and turns so 50 ep were just enough for it. To be a little more honest the last 5 eps may be a little slower than the rest and could have been fit into 3 eps (one of the reasons to that i didn't rate 10), but the rest 45 eps were not slow imo. each episode ending with such a turn that I couldn't wait to watch what is next.
the story starts when the main lead was 4 years old, therefore the story focuses on their parents and how everything happened. I know many people avoid long flashbacks, backstories, but this was really necessary for us to fully understand the characters. Also some say that the first half of the drama is slow while the action starts in the second half. for this I want to say that the first half of the drama is where all the knots are getting tied and secrets appear, while the second half is where they start to untie the knots and unveil the secrets. Therefore the second part would have never been there without the first. Personally I liked the first half of the drama a lot. As much as the second. The story continues on and we see the the main hero as 10 year old, and here again many tragedies happen. After this we go on to were our lead is in a high school (this is where adult actors show up), again they show us little part of their past, because this is were the lead decides what to do in the future. All these stories were so interesting and so freaking heart-wrenching, I cried my eyes out during the whole backstory. Because it shows us a never ending unfairness, powerlessness and dark tunnel without a light or exit. The villains are super annoying and so bad that makes you wonder how can people be like this. The story goes around a coward that tries to save his children. About betrayals, greed and unfairness. But among all of these we see such a nice, kind and warm family that makes you want to cry. From the 10/11th episode they finally grow up into 29 year old young people. This is where we finally see how our hero becomes a Thief (Robin Hood, Hong gil dong, Kaito Kid). The poor kids grew up and many set up on the path of revenge. And slowly they start to learn about the present, and about their past, and about the people who fought for independence in 1984 or something.

Genres: I wanted to address this too as I saw many comments saying there are too many genres in this drama. From one perspective yes, this drama is suspense, action, crime, family with a splash of history, romance and comedy The main characters are prosecutors and lawyers, policewoman who became an investigator in prosecution, Thief, and villains of a conglomerate. It might also be a little of a makjang drama, because all of the birth secret and stuff and the villains' families. They also sing here couple of times. The romance is not a cliche love triangle. it starts as one but then it changes and we see that there is no room for triangles or squares, because the main characters are too much in love with each other! (btw I am also in love with Jang Dol Mok the lead <3) And finally the comedy is a nice breather among the heavy stuff, as Dol Mok says let's laugh a bit, even if like this. (kinda sloppy translation from Korean)

Characters (Acting/Cast): The Characters where sooo complex in this drama. We see their whole past so we can see how many sides they have, and there is so much character development going on! Let's start from villains :D They were amazing! The Hong clan (LOL) - Pro-Japanese family that tries to find the ancient artifacts the Heroic Corps have hidden. Hong il Kwon - a very very bad old man! funny how he changes a little at the end. The older sister Hong Mi Ae and her son Yoon Ho.....(I will not talk about the husband) Omg.... They were soooooo annoying and super stupid at first, I hated them so much during the whole childhood part... kicking, beating, bribing, yelling.... gosh... however when Yoon Ho got older I started to like the character, and by the end of the drama he was great XDD haha so funny XDD same goes for the crazy Hong Mi Ae who was like super bad and annoying, but by the end of the drama I started to really like her parts as they were a very funny comic relief XD lol. The next are the little sister Hong Shin Ae, her daughter Hwa Yeong, and finally her husband the chief prosecutor Yoon Joong Te! they started of as dignified family but in the end couldn't escape the greed that run in the family. Hwa Yeong is your average rich mean girl. I won't talk much here. I want to applaud for Yoon Joong Te as he did a totally amazing job as a villain! and the last scene with him and the gold bird was great. It had a really nice message! There are also small villains like Choi Tae Seok and Choi Kang gyu who are the kind of people who do stuff for money and all. But in the end both of them played a big role in the story. I totally hated Choi Tae Seok at first but later I started to kinda like him. Choi Kang gyu was annoying, stupid and poor XD
Now to our good characters :D First of all is our dear main hero Jang Dol Mok/Kim Soo Hyun!!!! Standing ovation to Ji Hyun woo! To be honest I didn't like him at first. during the childhood part I thought it's a girl, and it was a shock to learn he is boy XD I also didn't think he will be the main lead. XD And the adult actor... I saw him in queen in hyun's man and didn't like him, I also saw him in angry mom (here i liked him though). But as the drama went on... I fell in love with the character Jang Dol Mok!! A Perfect man! As someone said he is the type that attracts women like a honey can attract bees ROFL. After going through so much he grew up so well! Such an affectionate man! His super warm relationship with his father and brother and others is just very sweet!! <3 <3 He has sooo much different licences! and is very strong! so precious! <3 My Ideal type!! <3 I can talk about him all day long but i'll stop here XD next is Jang Pan Soo and his wife! :D the father of jang dol mok, this actor received an award for this role! he was amazing at portraying a warm and kind coward, who cries a lot. The whole drama starts with him coming out of prison and going back to his family. He is a small town thief, that stole a little bread to feed his family, but got caught. he is one of the descendants so he tries to apply to some kind of charity foundation that can give money to the descendants of the heroic squad. However he gets himself in a BIG trouble after doing this. In the end he becomes the foster father of Jang Dol Mok. I havn't seen such a warm cute father-son relationship is a drama yet. Now Han Joon Hee prosecutor or Jang Mi Jae. the childhood version of him was played by Nam Da Reum, who is everywhere XD Han Jonn Hee's adult version was that kind of person, who does what he believes and is very very stubborn. However as he learns the truth he changes for the good. he still fights for what he believes in, but now he joins his brother to uncover all the crimes that the bad people were doing. I started to like him very much later :D The brothers relationship is also one of the most warmest relationships i have ever seen. <3 <3 I want to say just a little bit about the side characters. The grandmother and Master Kwon that appear during the high school days of Jang Dol Mok are like cherries on a cake at that time xD gosh, the appearance of Kung Fu master was kinda strange and funny at first XD but it was really refreshing and they became the characters I love. <3 San Woon Yul becomes the house where Jang Dol Mok and his father and many other people start to live and become a very cute, warm, big family! <3 . Shorry plays Dol Mok's friend who helps him with work. he brings comedy to this drama. And we also get many other side characters that play a big role in the whole story. However sometimes the appearance of those characters is quite random, that makes you wonder if they are really necessary, but they indeed are! I will not talk about them much so that it won't be a spoiler xD. Next is the father of our female lead, detective Kang seong il! He playes a big part during the backstory. a corrupt detective that does everything to raise his cute daughter alone, however he gets himself into a mess while investigating a certain case. Now finally onto his daughter, the female lead, played by SeoHyun (her first main role), Kang So Ju! yes, her name is after soju lol and she like soju. I think, many don't like the romance in this drama because of her, also the whole drama ... well.... She was great during high school, a strong, happy girl with glasses. and seohyun played her well. the problem starts later XD While being a 29 year old girl So Ju keeps being strong, loud, happy girl, just without glasses. That is her character and I don't think that the problem was the acting. the acting is ok. But this character get super annoying. There actually people who didn't find her annoying at all and liked her, so I think you should judge yourself, after watching the drama. For me she was very annoying and always doing what she shouldn't do, her way of thinking resembled han joon hee prosecutor at fisrt, very stubborn and righteous, thinking that law is everything and everyone needs to protect it. She is right actually! very right! however the law couldn't do anything against all those powerful people. She really gets in the way and all her decisions are stupid. Maybe all of that came from not knowing a lot. Because later when she learns all the truths and after getting in the way really hard, she starts to change!! :D she become less annoying, and finally starts to help. she becomes a really good partner for our Dol Moki. the thing is that for me her character was so annoying in the middle of the drama, that it made me think that she doesn't deserve Dol Mok, and seeing how she changed for the good didn't make me feel satisfied with the character, but somehow relieved. You know, like thinking: "Gosh, finally she does something normal!!!" . This is only my opinion however, you might really like the happy, strong So Ju. :) It is more like a warning in case you don't like her. XDD So wait till the end, and she will become good. XD
Music: Well the music complemented the drama a lot. It gave that old feeling of 90s at first and later. I also liked the songs the characters sang during the drama. The song that father and sons sang, also the song that so ju's father sang her, and then Dol Moki kept on singing to her (gosh he also sang and played guitar, and cooked well, and played chess and.... omg i need to stop)
Rewatch value: I don't usually rewatch dramas, but due to complicated-ness and richness of the story I gave it a high rewatch value.

Overall: This drama was a real roller-coaster. With many characters, many genres, many twists and turns. It was paced really well. The story goes on really fast, not slow at all. I suggest watching it when you have time, because watching it with just 2 eps a week might make you feel it as a boring drama. The first episode might be a little slow at first, but it gets intense from the middle of it. However prepare yourself as the first 10 episodes are super sad, the high school era has a little bit of a bright side, but it still has super sad and unfair moments. I really cried a lot. Later it gets better, because our kids grow up and are no longer those powerless poor little children. They become strong powerful adults with very kind and warm hearts. And the real action with the thief "J" and all starts from 11th ep. From about 25th ep they start to learn the truth, so the fight gets more intense. However romance also starts to bloom.
To add one more thing that maybe a little of a spoiler!! So go down to the next part, if you don’t want it. In this drama they also emphasized the respect towards women during the wedding scene. In traditional Korean wedding woman bows 3 times and the man 2. But here they did it so that both of them will bow 3 times, saying that in 21 century men also should show respect and not only women, to show that they will become friends and companions to go together from now on. I really liked it!

And finally the ending is satisfying. :D they finished all the story-lines they started and showed the "end" of all the main characters and some main-side-characters.

So if you are wondering to watch this drama or not, give a try to about 17 eps and only then decide to drop it or watch till the 50th. I don't regret the time I spent on this. It has been a day since I finished it, and I am thinking about all the characters, especially Dol Mok, and don't want to watch anything else yet.