This movie has appeal on three levels.
1. Honest Portrayal of Girlhood and Maturing Female Friendships
There are about a kimchillion movies that explore and celebrate male friendships, but honest portrayals of women and their relationships with other women are pretty hard to find in film (the most recent decent girl movie... Bridesmaids?). I believed in the characters of Sunny right from the start. The friendship and interactions of the members of Sunny were believable and honest. That of course made the whole movie more relatable and entertaining.
2. Sweet Dual Timeline Accompanied by Awesome Soundtrack
This movie achieves the perfect balance between it's two equally awesome timelines, the main character's present and high-school lives. Neither one outshines the other. Both timelines are entertaining and engaging and integrate well into the whole of the film. Additionally, the high-school timeline has a pretty awesome soundtrack (both Western and Korean).
3. Sense of Humour
The writer of this film obviously has a great sense of humour. I will direct your attention to probably my favourite scene from a movie, ever.
See what I mean?
The only downside of this film is the ending which totally abandons the honest feel and resolves the problems of all the characters deus ex machina style. Lame!
Oh well, still one of my favourite movies ever. Go watch it. Right now.