In fact, I was really enjoying this film for about three quarters of the way, mainly because Hathaway's character (Andrea, or “Andy”) was so engaging, and also was on screen for every second of it. She also has a very strong supporting cast in Meryl Streep, who seems to have fun with her villainous character, and Stanley Tucci, who at first seems equally evil, but later we find that his attitude is merely product of the industry, and we like him as well.
When you decide to make the main character irrefutably likeable, however, you had better make sure that the character refuses to do things that an un-likable character would do. "The Devil Wears Prada" does not seem to have any negative stance towards any of Andy's actions, even though she becomes, for much of her screen time, a product of the fashion industry that the film apparently reviles. It is unavoidable in a parabolic story such as this, to avoid some give to her nice-girl regularly-dressed persona, but the film should view this in a negative light, and it chooses not to. Perhaps the casting of Anne Hathaway, while definitely a capable actress, is a mistake because you can't help but sympathize with every bad decision she makes. When Terry Gilliam casts Jonathan Pryce in "Brazil", he is taking the chance that you may not agree with his protagonists' motivations and you may dislike him, but you still understand him and that's the point. A character that does immoral things doesn't have be likable in order to find audience recognition. They only have to be given realistic motivation for their actions.
The purpose of "The Devil Wears Prada" is to entertain, I suppose, and at doing that, it succeeds in its own modest way. It's funny and sarcastic, and probably true to the novel (although I confess, I never took the *ahem* time to read it). It's just quite uncomfortable when my personal feelings about a character differ from the film's, especially when the film is the one that sets the parameters.