Not being funny but I only ever see Dan Brown mentioned in a mocking tone. I've genuinely no idea, but are the books actually good in some sense?
Read them for what they are (fictional novels with allusions to truth and fact) and you will truly enjoy a good story!
They are very entertaining stories, that's why they're so popular. If that's what you're looking for then you'll probably like them. If you're easily annoyed by plot holes or historical/scientific inaccuracies then you might not, and if you're looking for sophisticated or artistic prose then he isn't the right author. Obviously "good writing" is subjective, but I think most people would agree that Dan Brown's writing is relatively simplistic, but that often isn't a problem when the story is good.
His books are perhaps in the same category as Nickelback albums: people love to rag on them, but if you look at the sheer number of units shifted, clearly lots of folks enjoy them.
I just read Angels and Demons. My take is that it is quite gripping and entertaining, and has no other virtues. The prose is just ok, and everything built above that is increasingly nonsensical. However, I'll endorse burkaman's reply as an equally accurate and more charitable review. :)
Sorry for the late reply. When I was studying my A Level physics me and my teacher read Angels and Demons at the same time. It is a silly story really but just scratched that sci-fi bit of my brain in the right way at the right time. It is pulp fiction but sometimes that's just what you want.