I really loved the originality of this story, it was also so poetic and romantic. The actor playing Go Yoo Han was so magnetic and attractive and Choi Yeon Woo's actor played so good, he can really make us feel his emotions. Both incarnated their characters so well and at some moments the atmosphere just went magical.
Some people thought the effects added were too much but I actually liked it, it is part of the charm I think.
The only bad point is the fact that episode 5 and 6 (I think?) were rushed and skipped important parts, I had to read the comments explaining some points to realize it made sense. After that, they managed to get back to a good rythme but because of that there was some inconsistencies.
Not all the story have been developed so I hope their will be a second season !

PS: It is so good to see korean industry getting interested in this genra because it is qualitative. Luckily, the episodes were longer than usually because it would have been impossible to keep fluidity otherwise.