http://www.telegraph.co.uk/culture/books/booknews/6194031/The-Lost-Symbol-and-The-Da-Vinci-Code-author-Dan-Browns-20-worst-sentences.htmlThey're pulp novels so it's not like you expect them to be great literature, but Dan Brown is particularly bad.
But maybe I was a little harsh, so let me refine my opinion a bit. (Minor spoilers ahead)
Angels & Demons and Da Vinci Code were interesting as they focused on a secret history, and revealed things about Paris and Rome that most people probably didn't know about. At the very least, it was stuff that only a serious history buff would have heard of. And it's shrouded in enough mystery that the premise is at least
plausible. He's offering up conjecture as fact, but the conjecture is as good a theory as any. The basic plot where the protagonists ran from one unlikely place/event to another by the sheer power of coincidence is another matter - but whatever, it's a pulp/action novel, I can forgive that.
The Lost Symbol was awful because compared to Paris and Rome, Washington DC just isn't that interesting, and there was nothing to "reveal". The Capitol building is based on Roman architecture! You mean that wasn't obvious to anyone with eyes? The white columns are kind of a dead giveaway. Further, the fact that the founding fathers were masons isn't all that secret. Nor are the masons themselves; they're not a secret society on the level of the Priory of Sion or the Illuminati. They're mostly just an old boys club with some wacky rituals. Most telling was the fact that the action wasn't driven by actual things found around DC (whereas the action in the previous two was driven by actual works of art and places around Paris/Rome) - it was entirely driven by an entirely made up pyramid which led to a "reveal" which was, frankly, stupid.
Digital Fortress and Deception Point didn't have even that much going for them. There were numerous plot points in both that just flat out made no sense, or were so unbelievable as to defy suspension of disbelief.