I wasn't expecting much of this one from reading the summary, or even from the first chapters, since the beginning was kind of slow and boring. But surprisingly, it began to impress me more and more as it went on. Menga went from being dull and wimpy to someone I could relate to, Hanako from annoyingly chirpy to a character with a lot of emotion and depth, and Choi Taek Gong or whatever his name was totally grew on me. I have a weakness for those bad-boy-but-not-really-bad-hearted types. The other two guys were sort of blah, but they served their purposes in the plot.
Anyway, there's just something about this manhwa. It's really funny at parts, yet also can be very melancholy and deep. I liked the slice-of-life vibes I got from it; like everything that was happening could really happen in real life to someone, and that they would feel what the characters were feeling. Also, unlike most manga/manhwa, I actually thought that the things the characters said were things that might really be said by people in their position; the dialogue wasn't weirdly poetic or too "quotable", but it was still meaningful and insightful.
I actually really liked the ending. Maybe it was a bit abrupt-seeming, but everything was tied up so it wasn't out of place. Also, I loved the final, brief scene - the way the characters interacted surprised me for some reason, but it made me feel happy.
Anyway, I'm not usually a big fan of manhwa, and the art here wasn't my favourite, but I loved H20: its characters, its atmosphere and its surprising depth of humour and drama. Give it a try; it's well worth the time.