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.

Read them for what they are (fictional novels with allusions to truth and fact) and you will truly enjoy a good story!