The Twilight was different but kept the spirit, mood and magic of book.
New Moon in the middle part was close to book but finally lost the main point: Edwards and Bellas inability to be separate.
Eclipse changed the characters and their motivations.
It seems so far that they will change the Breaking Dawn a lot. They said it in interviews that second film will be more about Jacob and Nessie. Of course, it was some time ago before script was done so maybe now they changed the conception.
I do feel that the movie's got better as they went on. I do still love the first but it was a little weak.