Doc, I've never read the book but when I heard the premise I recalled all the shows Art Bell did with the authors of Holy Blood, Holy Grail. It always sounded like a straight rip-off of the basic premise.
And I've heard opinions that it is not very well written and boring to some readers.
Of course, other people find it a good read.
My opinion: the movie could be a real dog if they start to water it down too much. One of the big stinkers. They've got to have a plausible villain or the conflict won't be believable.
People who like bestseller thriller type stuff seem to love it, as do people who claim to have more high-minded tastes who enjoy the Catholic bashing. Crap is crap, and crap wrapped up as some "expose of FACTS about the church" makes for a bestseller for all the Christian-bashers out there. They can read and enjoy their little fantasy and claim they are reading it because it has a lot of factual info, almost all of which has been completely disproven by the New York Times, even. Brown not only gets his theology wrong, but the head of some art institute wrote a piece for the NYT that showed Brown to not have a clue about what he was writing about in terms of the artworks he discussed.
My favorite error: Brown claims one of the figures in the painting is a woman, when someone who's taken the most basic art class knows that DaVinci and other artists commonly painted men with long hair. I mean, come on!