I think it's fair to say that this drama was good overall: the story is intriguing, well written for the first 10 episodes and it made me curious.
If I can say something bad about it, I'd have gone for a shorter version. 20 episodes are too many, since part of the story (expecially from episode 14) was just fillers.
I'd rather watch 10 very good episodes than 20 just for the sake of carrying it on.
The ending it's not that bad, but it's way too predictable: if you're not into open endings, just forget about this drama.
Onto the good stuff:
- Lee Seung Gi is just gold in this series, I cannot think of anybody else for the role of Son Oh Gong. Same goes for Cha Seung Won (Ma Wang) and Sung Hyuk (General Frost).
- Soundtrack is Bomb: "Let me out" is a hit, was stucked in my head for weeks, as well as "If You Were Me" and "When I Saw You".
- I loved the fact that supernatural elements, history and human nature are intertwined in the love story: it makes it really special.
- I don't particularly like Oh Yeon Seo, but i loved the first 5 episodes because of the tension between the two leads.
- Lee El (Secretary Ma) is one of the most underrated character, imho, but I loved her a lot.


I don't think I'd rewatch this, cause it's too painful now (i didn't enjoy the ending) and also because of the dragged out part (13-17), but It was definitely worth my time.