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.

See also: https://www.telegraph.co.uk/culture/10049454/Dont-make-fun-o...