I only saw the first movie, and that was ages ago - while I certainly didn't think it was a stellar performance, I do think that Kristen Stewart was quite true to the character in the book.
Oh wait, I did see the second movie too! In the theater yet. I couldn't stop giggling during the bad special effects of Bella hallucinating Edward underwater. I got dirty looks from the Twihards.
Full disclosure - I wanted to smack Bella upside the head and tell her grow up for the first three books. Last one was better because half the book was Jacob's narration and Bella was less annoying after ... oh - don't want to spoil anything. Just because I didn't enjoy the books, doesn't mean I begrudge anyone else their enjoyment!