This drama has a fresh, intriguing story; it's not just your ordinary rom com, it also has elements of melodrama, thriller and sci fi. The mix of genres was done expertly. As I watched this I got distinct echoes of Goblin here and there, and this was certainly not unpleasant.
The story was executed very well, the love story is exquisite, the way it begins and progresses is all very satisfying, the ending is very well done too. I must point out though that there were certain plot elements such as the mechanism of Do Min Joon's ability to return to earth that were not explained satisfactorily, also the fact that kissing made him ill, that was never explained enough and we're never told whether it was resolved in the end, and for that matter how will they have kids if he can't even kiss her without falling ill? anyway I mean to say that these plot holes were slightly irritating but I was able to suspend logic and enjoyed the show very much.

The casting and acting in this show is excellent, I believe that's it's main advantage even more than the story itself. the main leads are spot on, Kim Soo Hyun was suitably grave and stuffy with flashes of childish pettiness here and there which is absolutely fitting for a 400 year old alien who has had very few human relationships and no romantic ones.
Jun Ji Hyun for me was the star of this show, why isn't she in more dramas? She is absolutely perfect in this role, I'll even venture to say that Cheon Song Yi gives my all time favourite kdrama female lead Kim Sam Soon a very serious run for her money. I enjoyed her acting so much, her vulnerability, her maniacal egotism which reminded me of Dokko Jin was hilarious to watch; I enjoyed all the little monologues she had whether internal or external, I especially loved the way Cheon Song Yi had many moments where she displayed a great depth of feeling, when she expressed how much she loves Do Min Joon either to himself or to other people, she manages to transcend her goofiness and articulates her feelings in the most mature, insightful and heartfelt manner.

The second leads are also very good, I find Yoo In Na almost the same in every role she plays, she's a good actress but I don't see a lot of range in her abilities, maybe that's why she's usually the second lead.
Park Hae Jin on the other hand is a revelation, I've never seen him in anything else so I was very pleased to discover him here. His portrayal of Lee Hui Kyung is very good, for once the second male lead serves to drive the story forward and not to put spanners in the works just because.
His evil older brother was also well acted and was suitably villainish.
The supporting cast was great, Song Yi's family is well portrayed, the detectives are good, and Se Mi's mother also adds a nice touch to the story by providing a contrast to Song Yi's mother.
Overall the casting is a 10 for me.

The music is not prolific, there's just one song that stands out, but that doesn't affect anything for me because my attention was always focused on the story.

Rewatch value is 8, I'm not big on rewatching shows so this is very high coming from me.

Overall is 9, this is a great drama and I only regret that I didn't watch it sooner.