The dialogs are great, the story interesting with nice twists and turns Each scene is well constructed and fitted in a nice ensemble. The characters are well developed even if they're still single dimension. Finally, it has enough laughs to keep us smiling.
One particularity that is pretty interesting and would have benefited from being exploited a little bit more is the parallel drawn between girl-world, real-world, and animal-world.
There's very fews things negative to say about it. First, it has the teen movie vibe, however, it's the theme and it's not pushed to an extreme. Second, Rachel McAdams, the hottest girl, the queen bee, is not that hot as a blond, but she's believable. On the other hand Lindsay Lohan is very hot.
I say it's excellent light entertainment, with nice bits to make you think.