It is 100% impossible to make a movie true to a book, especially one this long. The movie would be like eight hours long, because you have to remember that in a movie we see parts we don't see in the book, it won't just follow Bella around for two hours, we'll see Edward in Alaska, we get to see them hunt James, we get to see James' coven before they meet the Cullens.
So we have to throw out all our unreal expectations, like expecting the movie to follow the book word for word, won't happen, will never happen, could never happen.
So all we can fairly expect is for the movie to follow the basic storyline, and include most (if not all) of the important scenes.
Plus, I've heard that Stephenie is quite fond of the movie, and we all know we wounldn't be fond of a movie that completely butchered our literary works.So I'm pretty confident that if Stephenie likes it, it will be a good film. And Catherine (the director) is a pretty big fan and I doubt she would murder something she loves.
I have confidence (especially since the second trailer came out) that this will be a good movie, and it will stay true to Twilight. (Plus, think of the riots it would cause if it sucked . . . I know I wouldn't want to be directing it . . .)