I liked the book. I liked the film. I found the film to be rather faithful to the book. A time-compressed telling of a detailed page-turner that actually works.
My one beef: There was absolutely, positively no good reason for them to have changed Sauniere being Sophie's real grandfather. This actually did the story a great disservice. In both the book and the film, Sophie was emotionally scarred from having witnessed her grandfather in the middle of a sex rite. But in the book, this is germane to the plot, as either her grandfather or grandmother (we don't know which) was part of the actual bloodline of Christ. Therefore, their coupling around the time of the equinoxes would have been celebrated. In the film, these flashbacks were never expounded on and therefore, without the explanation AND by changing Sophie's grandfather to being an adoptive grandfather, Akiva Goldsman really screwed the story up.
Also, I would have added in a single line when Teabing first defends the honor of Mary Magdelene. When Sophie repeats the long-held misconception that she was a prostitute, Teabing simply said "She was no such thing" without backing up his assertion. While the book expounds on this for a couple of pages, including the fact that even the Vatican reversed the edict in the 1960s, the film doesn't touch it. A simple line could have distilled all of this down to a few words: "Re-read the story ... Mary Magdelene doesn't show up until several verses after the prostitute's story." It would have probably even sent neophytes to the story to their bibles to see it for themselves. (Personally I've known about this misconception far longer than Dan Brown wrote about it so I wish more time had been spent on it in the screenplay.)
Other than that, I had no beef with what was added in and what was taken out. The subplot of Langdon falling into the well when he was a kid was great from a theological perspective. The notion of immersion in water, praying to Jesus, and ultimately being saved was not overt, but I got it. Also, I particularly liked how Langdon was a little more skeptical of the Magdelene/Sangreal theory, as he played the part of the masses, who are given no real voice in the book. He either brought up or answered many of the legitimate factual criticisms about the book, making the script that much more airtight. For example:
1) Constantine did not pick the books of the bible, the Council of Nicea did.
2) Jesus' divinity was not a "new idea" brought forth by the Constantine or the Council. Many held Jesus to be divine. Many did not. The Council merely codified the notion. But it was certainly believed since the time of the Resurrection, not just the council.
3) Opus Dei is not a villain; Aringarosa merely uses his Opus Dei to carry out his dirty work. The people of Opus Dei are strong believers in what they are doing. But I'm glad they pulled no punches showing the kinds of corporal mortification and blind adherance to the will of the clergy that Opus Dei are willing to commit.
4) They even handled the admitted hoax of the Priory of Sion very well. In typical conspiracy theory fashion, the film addresses the "hoax" as exactly what "they" want you to believe. Simple, but genius. It gives the skeptic nowhere to go other than faith.
On the whole, it's a very enjoyable film. Not earth-shattering (at least, not to me, because I've studied these plot points and lost gospels academically for far longer than Dan Brown has been writing), but certainly not worthy of the bile the press has spewed its way. Casting was excellent, Direction was sharp, and the writing was above-par for Akiva, whose dreck I usually cannot stand in the least. No Oscar material here, but worth the price of a ticket nonetheless.