Dan Brown's book The Da Vinci Code has certainly got a lot of attention. And criticism. Of the criticism, I find most interesting the criticism of his writing style. As in, word and phrasing choices.
A good well written example of such criticism is a post in The Language Log titled The Dan Brown Code. It mentions specific points that make it bad writing.
I can't argue with those specific points. I can't argue that they are good, that there's a good reason for Brown to have worded things as he did.
However, frankly, for me as a reader, it doesn't matter. I liked his writing style. Those things which are criticised didn't bother me. They didn't attract my attention, and thus didn't distract from following the story, a story that I did enjoy.
It's all a matter of taste. Different people have different things we like, dislike, or are neutral about. Dan Brown's prose writing is not a negative for me. For me, I'm neutral. When reading the book, it didn't attract attention negatively or positively. For me as a reader, it did the job of conveying the story. For others, it doesn't do that so well, and that's okay.