If you're looking for a movie to compare to the books you'll be disappointed. There are scenes added that don't happen in the books, there are scenes that should have been in the movie that weren't. They also changed a lot of minor details, which is annoying if you notice, but they don't really affect the integrity of the story.
If you're curious about the movie based purely on its composition, it was wonderful! The special effects were great - check out the film with digital projection. My only complaint was that they made the ending kind of anticlimactic - there was a lot more chaos in the book that they could have played up on.