This drama was so good. I've read many reviews saying either the first half or the second half was better and the rest was kinda crap but I just can't agree. I tried to search the flaws in this drama so hard but I couldn't find any. I guess everyone has a different type, that's why I'll try to be as objective as possible now. I don't know if this will be your opinion, but give it a try!
When I first started this drama I was very torn whether to continue or drop this drama. Yes I'm speaking the truth even though my rating might make me look like a liar. Halfway through the first episode I asked myself if I shouldn't watch a drama of which I am sure that I will like it. In the end I watched two episodes of this drama and I don't regret it. In my opinion, that's when the drama started to climb the hill, slowly but surely. (I started to like it after 3rd/4th episode)
First of all, the story: Since you should've read the plot summary already I won't talk as much around the bush as I just did. It's a fantasy story which can be liked even if fantasy usually isn't your type. Because I speak from experience, I just can't stand fantasy because it makes a story so hard to feel realistic, but this oh this drama felt so real. The plot twists were what I liked the most. When you think the situation finally calmed down another event takes place and it's just entertaining to watch this story. Many people said something about fillers and I do agree, but to be honest those fillers were what made the story even better. Otherwise it would've been a short dramatic ride.
The cast did one amazing job! Because this is a fantasy show I had a hard time to get used to the characters but the cast's acting made it really easy! They acted so natural, as if it's the most normal thing to happen that you talk to a god and the next second to a demon or whatever. All, and when I am saying all I mean all roles were played well. Even the most unnecessary ones. And the characters lived through a development that every drama wishes to have. The people from the first/second episode had nothing in common with the ones in the last. I mean, they were both perfect but it was just fascinating how good and realistic the character development was made. And not to mention the chemistry between the main leads. I loved every relationship there was. But Son Oh Gong and Jin Seon Mi aka Sam Jang were really really well played. Both were extremely hard roles but pulled off by pros!
Next is the music. The OSTs are reason enough to watch this drama, I'm not kidding. Every single OST made my heart melt like I don't even know what. I can't find a better word as perfection to describe them. And they also fitted every situation very well. Not only OSTs but also random background melodies were so well produced and fitted perfectly.
All in all this drama was a ride of emotions, it could break your heart in one second but the very next you had to pause the episode because you can't stop laughing.

Maybe this review is so much better than others because I expected nothing from this drama. But maybe you just look into it and find out for yourself if it is wort it. But however you decision might look like, in my opinion it is a must-watch drama. Even though some people didn't like it I'm sure they still were glad that they watched it. But please don't judge the drama by not having seen everything! I was about to drop this drama after about 40 minutes and now look at my ratings.
I hope you will watch this and most important enjoy it at least as much as I did!
Whatever your choice is, have fun!