Honestly the positives mentioned below are mainly my thoughts for the first 6 episodes of this show. Once we hit episode 7 it began to lag a lot and lost most of it's charm.
It was soooo much fun at the beginning- surprisingly so. I was not expecting much to be honest, the rating on MDL is a little lower than most dramas and I haven't seen it be talked about elsewhere.
Lee Joon Gi and Shin Min Ah have fantastic chemistry, which I also wasn’t expecting. I love Shin Min Ah and am… neutral on Lee Joon Gi. I like him sometimes (My Girl, Flower of Evil), sometimes not (Scholar who walks the night)- but I think it's all on direction or the script because here his character has such personality (opposed to his usual more serious roles). He really shines here, and I believe Shin Min Ah heightens that as well due to their chemistry and ability to goof with eachother convincingly.
Yeon Woo Jin is great as always too. Had an interesting character arc overall.
Hwang Bo Ra is sooooo cute in hanbok! I had never seen her in a sageuk before but I think it really suits her.
The character Dol Soe was insanely annoying, and it would have been an improvement to have him omitted from the show entirely.
I didn't care for the heaven/hell characters, or the villains (except when Yeon Woo Jin was involved with them), so it mainly became a continued watch for the characters I did like and not the story.

The plot is initially fun, plain and simple. When your lead characters are so likeable it's easy to make a simple story like this enjoyable.
It definitely would have benefited from being the standard 16 episodes instead of 20 though. It became very drawn out. I feel like they packed all its appeal in the first chunk and then we were stuck with the rest slowly occurring.

It got to the point where I was struggling along, fighting to finish, so during episode 14 I finally decided to drop it. It's a real shame because I was having so much fun with it at first.