WARNING: Don't watch this separately from part 2 if you can't handle sad endings, because it has a crazy angsty plot twist in the end akin to "red wedding" from GoT!

Other than that this drama has a great mix of genres - fantasy, drama, comedy, action. The world is intricately built, all characters are given enough time to introduce themselves and most lead characters have great character development. The plot is very complicated and twisted, with deliberate hints about late spoilers dropped for attentive watchers from early episodes. It will keep you hooked and guessing what will happen next without you actually being able to figure it out, so unpredictable it is.
However, in later half of the show for the sake of dragging it out to 20 episodes the plot started to stall and meander around the issues with no real resolutions until the last two episodes, where everything was dumped on the audience - a common problem of many kdramas that just have no sense of pace and don't know how to end themselves properly. Some sub-plots especially took an egregious amount of screen time, like that "fake Bu-Yeon" plot. The ending was shocking for sheer shock value with lots of unresolved dangling loose ends. The sins of FL, who's occupying the body of an innocent girl, finally caught up with her in the end, but said innocent girl was done dirty, she didn't deserve any of this. In any case, stay tuned for part 2 to see the real ending, they're not done yet.

The cast did a stellar job at portraying their characters. Special props for FL who played a difficult 3 in 1 role.

OST and production value of this drama are also through the roof.

Overall, an 8.5. A very solid and unique kind of drama. But I can't give it a higher score in good conscience because of the problems with the plot I highlighted above.