I will share my personal opinion:

This movie is overrated on IMDb. No offence but I found Korean humor presented in this movie to be silly & irritating(eg. the disgusting scene when the girl vomits on a person's head, slapping in public, and the girl who is a tyrant). May be it's the culture difference. Lets come to the romantic part. I don't know if it is a real story. But I could not connect with the characters in this movie. Although the girl is beautiful, she seems too complicated, dictating and kinda mentally ill to fall in love with.

Overall: "My Sassy Girl" is An average movie.