This was the first kdrama that I watched and enjoyed, which is strange insofar as I know that it's not, strictly speaking, good -- but it is so entertaining! The third episode is, truly, among the funniest things I have ever watched.
The show's strengths and weaknesses are kind of mixed so that there is not one single component that makes the show "good" or "bad". Taking them one by one:
1. The story is a little up-and-down at times; sometimes it makes sense and sometimes it does not; sometimes it's fast and sometimes it's slow. However -- the mood of the show (more comedic than dramatic) accommodates the inconsistencies. And the comedic scenes in the beginning of the show are absolutely brilliant.
2. The acting is a bit of a mixed bag. I admit to absolutely buying into and adoring the male lead and almost all of the supporting cast. The female lead and the Devil King are less to my liking -- but I think that a large part of that is the script / what they have to work with and the types of roles they have been cast to play. And there are scenes in which the FL and DK absolutely kill it!
3. The music: The songs in this are wonderful. The "dramatic" music sometimes is over the top -- but regarding the show as a comedy makes it work, weirdly.
So a lot of good mixed with some bad. But, really, I think that fixing the bad elements would take away from the lightness of the show and make it less entertaining.
Personally, I have already rewatched it a few times :)