Girlfriend is a manga which leaves me with mixed feelings. Very mixed feelings.
One one hand, the art is gorgeous, and a couple of the stories are rather sweet.
One the other hand, the female characters are disloyal, two-timing sluts who bring my blood to a boil.
Now, I'm no stranger to ecchi manga, and such material can be rather fun to read, if only for the sex. Girlfriend, however, goes beyond simple guilty pleasure.
The sheer hedonistic nihilism present in this manga is simply sickening. The problem isn't the cheating (there's plenty of cheating portrayed in just about every hentai manga) or a lack of a story - but that the story defends the cheating.
Some may see this immoral, and quite frankly disgusting, behavior as "realistic" and that it reflects the real world. So? That doesn't make it any easier to stomach. If you see the characters acting like..., well, 'whores' is the only word which comes to mind, how the hell are you supposed to sympathize with them? (And if you do, I urge you to seek counseling.)
And that's the problem - the author portrays these betrayals in not only a positive light, but seems to honestly encourage it.
Like another reviewer mentioned, the author could have shown the negative consequences of the characters' actions, and at least pretend that there was a lesson to be learned. But no, the only message this manga delivers is that it's perfectly okay to cheat on your boyfriend - you'll still get the prince (and a fuck-buddy) in the end.
Girlfriend is one of those mangas that shows a pretty face in order to hide the ugliness underneath. In the end, it fails to deliver a romantic (or even compelling) story for those who actually want to read the manga, as well as fails to deliver enough nudity for those who are in it for the sex.
Unless you are like the author, and like to see two-timing skanks being portrayed as the 'heroines', you'd do best to stay away from Girlfriend.