Actually, it's a pretty amazing drama.
Now for the things I liked about this drama.
Of course, the main lead girl - Jin Seo Min(Oh Yeon Seo) is amazing. Her acting is impeccable. I really loved all her outfits and her simple hairstyles in this drama. It was not over the top and very natural, showing even more how her character is well put together from the outside, but actually, it's just a facade. Jin Seo Min is actually really lonely and scared. She is feared by all people that surround her because of her abilities to see demons. So she grew up alone and separated from kids at school. No one wanted to be friends with her. And as she is now an adult, her only living family despises her and calls her a freak, unlucky girl. One thing I didn't like about her is her selfishness. She actually didn't think about the feelings of Son Oh Gong and how much he might be hurt by her trying to trap him. She only thought about herself. And you might think that a woman, who has went through so much shitty stuff in life at least would be considerate of others, but no!
Main guy - Son Oh Gong(Lee Seung Gi)...He's...how should I say it... Very different from other main lead guys in other k-dramas that I've watched. Maybe that was why it was so refreshing to see such new character and his persona. And I still can't forget about his iconic coat - it's amazing. And so is his style. In this drama all of the characters have amazing style which I love and adoooore.
Minuses. There was one annoying minus. Special effects. Why did they make all the magic and supernatural things look soooo bad? For example, when Ma Wang shows his real self by floating surrounded by fire he looks very comical and the effects are very badly done. Don't know, maybe that is the joke, maybe it was supposed to be this way but it bothered me a little bit. All the magic and non-realistic stuff are pretty bad, not going to lie. If you can look past all that then it's definitely worth watching it.

Now, a bit ending spoilers ahead:
The ending was anticlimactic, if you compare it to all episodes before. WHY? Because Jin Seo Min basically was not important at all. Through the whole episodes it was hinted that she will be the cause, that only she can change the ending,...but basically...She wasn't needed. And as usual, the guy saves the day and the world. Because we can't have a different ending where a female girl FINALLY DOES SOMETHING SIGNIFICANT!!!!(I'm writing this just as I finished watching this, so excuse my ranting). So overall... I didn't enjoy the ending. It was not...pleasant at all. Things that should have mattered didn't, and things that shouldn't - were made important...At the end...I was more interested in side character stories than the main lead couple.

NOW SHOULD YOU WATCH THIS DRAMA?
Simple answer - yes. If you are okay with so-so ending, but very cute episodes in the middle and series that happen in these times but also jump back to the past. If I knew the ending before, probably I'd still watch it, because chemistry between the main couple was very good and I loved side-characters.