If you've read the book, I think you'll be pre-disposed to liking the movie. Admit it. You want to like the movie. Movie versions of books always seem to gloss over the details. Twilight is no different. There's an adaptation or two, but nothing that will bother a reader too much. Casting choices were mostly good. Alice and Jacob were perfect. Bella was damn good, but the actress added a studdering sort of awkwardness that I didn't get from the books. Edward was okay. Jasper was too small and girly.
I fear that if you haven't read the books, you might not get as much out of the movie. But it's hard a reader to see the movie from non-readers' eyes.