I've seen the movie when I was back home, a couple of weeks ago. Now, I read the book quite a bit of time ago, but I think the movie was pretty faithful - sure, a couple of subplots and such were cut out or simplified:
for example, there was romantic tension between Langdon and Sophie, and if I remember correctly Aringarosa was not acting for the Vatican, but by himself after hearing that the Pope would withdraw from the Opus Dei. Additionally, in the book it turned out that the man who had seen Sophie and Langdon entering the church of Roslyn was her brother, and that the curator of the Louvre (who was her true grandfather) had decided to split brother and sister after the death of their parents, fearing it had not been an accident, and had left for Paris, leaving behind his wife to take care of the boy.
However, the movie is faithful to the book, which was in itself hardly an action story, and far more of a "cultural" thriller. Hence the lengthy expositions of theories and legends, which fit the movie perfectly and are essential to the evolution of the story, even though they may seem boring or bothersome to those who are even just slightly more action-oriented.
Personally, I found it quite good. I like Tom Hanks as an actor, and Ron Howard as a director, and I think Bettany as Silas and McKellen as Teabing were very good too. I enjoyed the movie quite a bit, and as those of you who visit the Close know, I'm a Christian (or quasi-Christian

), and I can assure I didn't feel threatened in the least by the movie or the theories it exposes... it saddens me to think of all the fuss that was made over it.