I watched this movie on Netflix.
A good thing about 20th Century Girl is its pacing. There is no boring moment in the entire movie.
The story is a little bit similar to Indian Film, 'Mujhse Dosti Karoge', or any movie where love is a result of mistaken identity or miscommunication. However, a bittersweet twist is introduced at the end of the film which may attract contrary but equally strong opinions from viewer based on type of endings they like. Such endings are common in K dramas though.
The protagonist is really, really attractive looking. In fact, I started watching her K Drama 'Backstreet Rookie' because of her only.
All the characters have their witty scenes where they bring smile to your face, be it Titular Character's romantic and emotional best friend, her caring and love interest and love interest's sarcasting best friend, love interest's little brother, protagonist's typical classmates, strict and eccentric teacher, cool father, annoying mother, irritating little brother and even the blink and miss school doctor character or the blind date introduced near the end of movie.
The romance in the movie is subtle and natural, does not feel forced at all. This may be a simple but rare feature in any movie.
Despite all the good things as above, my only grouse is that the movie does not show you any extraordinary or iconic scene.
In conclusion, this is a two times or may be three times watch and can be recommended to your friends with you joining them.