So personally I'm a fan of the Twilight books, and while this film is much better than Twilight (not that this is saying much), it still waters the novel down to the point where it's rendered a shadow of its "paper and ink self." I actually feel kind of sorry for Bella-the-book-character because her depression is portrayed as solely the result of getting dumped and, as several reviewers have already noted, is doused with all these teen angst themes when in the novel it's much more complex. She's a young girl who's always related poorly to most people, and Edward and his family are among the very few whom she feels on the same wavelength with; they've essentially become her family and she's already made the choice to become one of their kind one day, and when he leaves her she loses all of that and to her mind, it really is "like she had died." To me that makes her emotional state understandable within the context of her one-track mind and marked lack of cynicism, but of course all that gets glossed over in the film and it's just this blank character with no ambition other than to be with Edward in some way. What's more, Kristen Stewart's empty expressions through it all made me gag, but at least they were somewhat better there than in Twilight.
Still, I thought Taylor Lautner did very well in both films, especially for his age, and all of the actors playing the Quileutes more than made up for Bella's lack of acting skills. However Michael Sheen took the cake with Aro: he portrays the creepily manic, greedy, utter-know-it-all of the books to a "T" and for me his hysterical laugh when Jane failed to torture Bella with her mind was the best part of the whole movie. It has its good bits and its decent bits, but the script is crap and sounds even crappier to viewers unfamiliar with the novel, since for most of the cast the most clumsily delivered lines come straight out of it.
All in all, the film is an okay rendition of the book, but far too manipulated to suit the commercial preferences of ninth-grade girls to appeal to much of an adult audience outside the Twilight fanbase.