Ok , I'm not really a manga reader, but here's my 2 cents about this one:

No central story (therefore the low grade), but keep in mind that this particular manga doesn't need one: It's all about the ups and downs of a bunch of (mostly) high-school relationships.The characters are explored in sufficient depth, provided their stories are usually very short.I found the couples very interesting, for each had its unique share of problems.I admit that many of the situations aren't terribly realistic, but it didn't bother me personally.
Many of the other reviews I've read complain about the fact that there is too much sex going on in the manga and that many of the girls act slutty, cheat , etc.While clearly there are some exaggerations, I find that overall the manga does a good job at depicting the psychology of the average teenagers throughout their first steps in the world of interpersonal relationships: sometimes too eager, sometimes too shy, excessively horny and psychologically unstable, girls can't figure out their emotions, guys can't figure out the girls, etc, etc.Everyone who's had at least one turbulent relationship in their youth is bound to get this good ol' "been there , done that" feeling at least a couple of times while reading this.
Bottom line, it's certainly not a manga everyone will enjoy, so make sure to give it a try and find out if it's your cup of tea