Kam and I saw The Da Vinci Code a couple weeks ago. A few thoughts and comments:
I haven't read the book. (Kam had listened to it as an audiobook.)
However, having lived in the US and paid a small amount of attention to pop culture in recent years, I already knew the central surprise. And even when I first heard the central surprise, I wasn't especially surprised, because my father told me years ago about the book Holy Blood, Holy Grail. (And btw, in case anyone doesn't know this, the real-world Priory of Sion was a hoax.) (Yes, yes, I know: "That's just what they want you to think!")
So although watching the presentation of the central idea was mildly interesting, there was little information (or faux-information) in the movie that I wasn't already familiar with.
At some point during the movie, Kam leaned over and whispered, "It's like watching someone else play Myst." I thought that description captured the experience perfectly. Certainly a better description than the one I had been working out in my head, which went something like "It's a two and a half hour long infodump, punctuated by occasional chase scenes." Which was basically what Roger Ebert's review said, too, referring to the formula of the book (replicated in the movie) as "exotic location, startling revelation, desperate chase scene, repeat as needed."
Ian McKellen was of course by far the best thing about the movie, as he so often is, adding charm and humor and character. Everything got more interesting and fun when he was onscreen.
I did think the movie had some nice moments, and some nice effects (especially the funeral scene for a famous scientist, he said, trying for some reason to avoid explicit plot spoilers). But overall, I didn't think it was very good.
I imagine most people already knew whether they wanted to see it or not. But if you're still trying to decide, you might instead consider renting National Treasure (which I semi-reviewed last summer), which is pretty similar in some ways--full of historical infodumps, conspiracy theories related to the Knights Templar, action scenes, puzzles to solve, twists, catacombs, etc.--but which is more fun. And I still think National Treasure would make a good Fourth-of-July movie for those who aren't feeling entirely cynical about the US.