I've seen this said before either in comments or reviews: When first watching this, the concept reminded me of Hotel Del Luna. BUT
I will say this is NOT an exact copy of Hotel Del Luna.
I personally don't care if there's similarities between two dramas/stories because I feel all dramas have similarities when it comes to their themes/messages. While the concept and some of the ideas are similar, these two dramas are not the same. The message of this drama was slightly different than that of Hotel Del Luna's. The ending was different and better than HDL as well. I feel like the writer did a good job of making this drama special in its own way despite having similarities to another drama. And I will say the acting was amazing as well.
Story & Acting-
If you follow the story well and pay attention, you will figure out or have a good guess for SOME of the things that are revealed in Mystic Pop up Bar (not all of them cause some things I guessed incorrectly, but the main "plot twists" I guessed already). You will already know it before the characters in the drama find out.
I don't mind that. I watched because of the interaction of Weul joo, Gui, and Kang Bae's characters. (Mainly Weul joo and Gwi). They are very entertaining to watch.
I wished there were more Weul joo and Gui / Kang Bae and Yeorin moments, but it's fine since they had to finish the story in 12 episodes. The chemistry is fine for a 12 episode drama that is trying to finish the drama's story, it would be hard to add more scenes developing romance. I actually thought there was a good amount of chemistry between Weul joo and Gui.

I felt the ending was rushed. There were a few things they didn't properly explain just to give it an ending but I let it pass because I was satisfied with the ending.
There were a few things I wanted to know, which is why this drama isn't exactly perfect.
This story, almost made me tear, definitely made me laugh, and the growth of the characters were decent.
A lot of people felt that the side stories were unnecessary, but I feel it was necessary just like it was in Hotel Del Luna. Through most of those side stories, you get to see the growth, development, healing, and personality of the main characters.
You see Weul joo who appears to be a cold person trying to reach a goal, but you see her real self later on and her motivation. You also see Gui's purpose and you just end up cheering them on.
I've seen Choi Won Young's acting in other dramas (Hello Monster, and others) and I already had expectations since he is a decent actor. He did not disappoint, he did a really good job in his role as Gui. Sung Jae didn't do so bad as Kang Bae.
As for Hwang Jung Eum, I've seen her other dramas, and was hoping she wouldn't scream too much in this one, and I'm glad. Her role is different and much better. She did a great job and yes she was gorgeous with those hanboks.