I liked all the main characters in the cell, be it the complaints-writing Dr. Go, the complete weirdo Loony, the lisping dirtbag Kaist, the mellow big brother Min Chul, the wrongfully convicted Jung Woo and the simple-minded, gentle Je Hyuk.
I especially liked Min Chul and Lieutenant Peang, both being thoughtful of others in their way. Paeng really surprised me because he was rather annoying with his constant cursing, but over time you can see his heart of gold.
The acting was great, the comedy especially of Loony with other characters was really funny and the emotional scenes conveyed all the feelings without being too heavy.
I liked the directing as well, especially the first 20 minutes were already such a pleasure. How Je Hyuk went in the tunnel and exited it in the same scene but in the opposite direction in another vehicle and in handcuffs, that was a really well-excuted idea. And Joon Ho's joy before meeting Je Hyuk after all this time - even with a spring in his steps -, but then receives another task, so every life is sucked right out of him. Atfer those scenes I was quite sure I would like this series, which proved itself true.

SPOILER
Episode 15 was such a rollercoaster ride:
There is Min Chul, this really nice person who committed a terrible crime. He really repented and displayed this with his spotless record. He is so warm and just a huge teddy bear. I really liked how he was so grateful for meeting his daughter and that he actually wanted his wife to be happy with another man. In the end he received the pardon and I was overjoyed.
Jung Woo's brother got to punch the bastard who was responsible for the incarceration of his brother. He really deserved that. And the meeting with Park's mother was something else. She is the one who suffers a lot; she lost her son and thought the perpetrator is behind bars and has to learn she cursed the wrong person. The guilt and suffering is visibly wearing her down tremendously.
To see how Han Yang (Loony) mess up his life again and to see the disappointement in Ji Won's face, was really hard. I know that many drugs alter the brain structure fundamentally, that addiction is a disease and that will power alone is not the solution. But I was so frustrated to see a character fail so hard and drag other people with him down. And it had to be the gay inmate. Why did the writers decide to give the drug storyline to a gay inmate? As far as I know, there are not many gay characters in Korean series intended for the general audience, so it is really annoying that they wrote him like that.
It is a magnificent episode, even if I have some qualms with it, and I'm really glad I watched Prison Playbook.
The one negative point is the ending of Han Yang, which was totally uncessary since they left him hanging in the air. He could have returned to his family and his boyfriend and be completely astonished that his mother actually came. Be thankful to his boyfriend to stick with him through all this. Or at least continue this storyline further, but this character did not deserve nothing after fighting so hard and than failing right at the end. To have just one cellmate be treated like this is quite unfair and of course it is the gay one.
SPOILER END

I want to end on a happy note: I enjoyed this show a lot, liked many characters and especially their comedic interactions were pure gold. I definitely recommend this series.