Broken Base: There were a lot of fans who weren't happy that the girls, sans Carmen (who ended the first book single), all ended up with their first love interests, arguing that it was unrealistic and didn't speak of much growth, putting them right back to a romantic square one (and it doesn't help that said love interests weren't allreceived well). Others find the idea of ending up with your First Love romantic, connects the stories, and falls in line with many young adult narratives.
Marty Stu: Win. He's Carmen's dream guy and a Satellite Love Interest who doesn't seem to have any real flaws. When Carmen comes clean about her dishonesty and tells him she can often be a mean, bratty person, he comes clean too. His flaw is that... he used to be fat. Even Carmen seems briefly baffled by this.
Romantic Plot Tumor: Good Lord, Kostos and Eric. They are likable in the first couple of books, but by the end, they've become nigh-insufferable in the way they interact with Lena and Bridget.
It becomes even worse in the sequel book. Lena is thirty years old and still obsessing over Kostos whom she hasn't seen since she was a teenager. At least Eric and Bridget have been living together.
Sequelitis: The first four books are pretty well-liked, but almost anyone who's actually read the sequels prefers to forget them.