I liked it better than I expected. I went into having just finished reading the book (I admit it, I picked it up at the supermarket and read it over the weekend. ) which turned out to be a good thing. On the whole, the movie doesn't deserve the panning it's getting from critics; as an action thriller it leaves much to be desired, but as a maguffin quest, it's a fine example of the genre. It's beautifully shot, and since artwork and architecture are an integral part of the story, the visual medium of film serves well to illustrate those aspects of the plot. On the other hand, much of the story is explanations of this or that bit of obscure history, or solving puzzles. Film does not lend itself well to "talking head" stories - but Ron Howard does an able job cutting away to flashbacks or other scenes while the dialogue occurs in voice-over.
All the actors turn in a fine, understated job. Dan Brown's books aren't about characters, really - they're plot-driven, and his characters are two-dimensional at best. The actors simply don't have much to work with other than being a two-legged plot-advancement device. At one point, Sophie (Audrey Tatou) remembers why she's on this zany quest after all, and has a little freakout - but it's so tacked on, we can tell that the script just doesn't take into account the interior lives and ambitions of its characters. The movie changes the motivations of some of the characters to make their actions more approachable, but even so ... we're presented with caricatures more than characters. Noteworthy is Paul Bettany (Silas) who brings humanity and pathos to a character that would otherwise be nothing but a stalking villain. His Silas has a melancholic and confused gravitas that renders him far more sympathetic than a more heavy-handed villainy would have allowed. Actually, Bettany is becoming one of my favorite actors, and not just becuase he was Dr. Maturin. (Though that too..)
There are, however, intrinsic plot holes. Why does Silas (in the opening sequence) extort the hidden location of the keystone from Saunniere - when he could have just as easily made him give up the location of the grail itself? There's another point, later in the movie when one of the villains (whose identity I shall not reveal) has everything he needs in his grasp - but irrationally fails to act. I can only assume that he does so because he knows a plot twist is coming, and doesn't want to ruin it. This is simply clumsy writing on Brown's part - his character's only motivations are to advance the treasure hunt, not to act like real humang beings.
Still, I enjoyed the movie more than I expected to - and again, it looks fantastic. If you've read the book you'll probably enjoy it more than elsewise, because a lot of exposition occurs fast and heavy. Even so, I recommend it. And I say that having been prepared to hate it - I really loathed "Angels and Demons" - and "Davinci Code" is a far better work, both on the page and on the screen.