Dan Brown has published a total of five novels till date and all of them follow a set formula: an academic for a hero, a pretty, highly intelligent heroine (who is somehow always clueless), and the obviously good-guy/protector/mentor who ends up as the villain. All this is tied up by an absurd plot. To Brown's credit, the pace of his novels are always fast and the language straight forward and simple, so you can finish his books on a long flight.
So getting down to his books:
Dan Brown writes wonderfully fast paced novels. They are complete no brainers and serve to make one feel smarter as they are full of "obscure" tit bits known to everybody over the age of twelve. Often the plot turns so absurd that reading it is a laugh riot. Anybody who enjoys 70s Bollywood masala movies and Salman Khan flicks will love 'em.
Plots are stupid, stupid, stupid. Language is nothing great. There is very little to learn, understand or explore here. Mediocre writing, no research and slightly pretentious. Why must one have "Facts" at the begining of every novel? Besides being stupid the plots are also simplistic and idiotic. Especially the ones with Robert Langdon as the hero. "A secret that will shake the world" is what is promised in every novel. None of these "secrets" are earth-shattering. Whether Jesus was married or not is irrelevant as are the obscure practices of cults. Most characters are stereotyped.
Facts are less believable than a bad wikipedia entry. He knows no science (not even basic 10th standard stuff. In Digital Fortress he compares computer viruses with bacterial viruses! Does he mean viruses or bacteriophages? Bacteria and viruses are totally different biologically, and this is basic stuff). While I am a technophile, my knowledge of programming and hardware is limited to QBASIC programming that I had learnt in school and stuff that I read in newspapers, but even I can find innumerable plot holes and absurdities in Digital Fortress. Even stupider was Deception Point. The Da Vinci Code and Angels and Demons were a race against time in a pretty European city. Thankfully, in Angels and Demons, at least there was a bomb to add some tension in the story.