I really enjoyed it. I don't know what people expected to be honest. People always have these quibbles with adaptations, but I would argue they always have unrealistic expectations as well.
Silus, Leigh, Neveu and Langdon were all well cast in my opinion, and as has been the early-word -- McKellen is particularly good.
A lot of the criticisms -- that it borders on dull for instance -- I just can't agree with. It followed the book as close as possible in a two hour sitting. I also read some criticism of Howard using historical-flashback style sequences and so forth, but I felt they necessarily abridged a rather difficult story reasonably well. Narrative just isn't the same in a movie as it is in a book. And besides which, it looked good. I liked how they merged imagery of old London with new London for example, I enjoyed the closing moments at Le Louvre, and visualising Langdon's thought processes on the screen worked as well.
There are people who go and see movies with their critic hat on -- then there are people that just go to be entertained. The Da Vinci Code is good enough to win over less jaded crticis, and less hardened fans of the book, and its good enough to entertain people.
Its not an incredible movie, but its an enjoyable one. I don't feel ripped off having paid 5 pounds to see it. Not in the least. I'd recommend ignoring the backlash.