I thought the movie was good. Yes, I read the book (although it's been a while) and the movie didn't capture every detail but really, how could it? Maybe some more background information would have been helpful but would have been Tom Hanks receiting religious history for 2 hours. I thought the ending was an improvement over the book, actually.
Even if you haven't read the book, it's an entertaining action movie that moves along quickly and makes for a good night out.