If you are SUPER romantic and have a thing for supernatural drama, you will absolutely love this one. And even if you are not, it would be nice to watch, basically, for the main cast. But if you are seriously anti-romance, this will get to your nerves a little too much to handle on a binge-watch mode.
All the male leads have done a great job. The female leads were also good. Maybe if Jun Ji Hyun didn't go over the top in some of the scenes, there would have been no flaw. Some of the moments which were supposed to explain a lot about Kim Soo Hyun coming to earth from the stars, staying for as long as he did, and what would happen if/when he goes back, did not deliver any meaningful explanation. Was that intentional to keep the mystery, I guess not, it left so much to be missed. But there were many points that would make up for what was missing, like the cinematography, the music, scenes and sets designs, all were very good indeed. And again, high marks for the cast.