I thought Twilight was absolutely fantastic - it was well cast, and followed the book almost EXACTLY!
There were 2 added scenes, I think, but they just added to the back stories (i.e. about James, Victoria and Laurent being the "wild animals" killing people) so that the movie wouldn't seem choppy. The cinematography was fantastic, and made me decide that I too want to live in Forks!!
But I definitly agree with those who said "Read the book first!" If you can, read ALL the books first, because certain lines of Edward's will have you begging for movie #4!!