The only novel by Dan Brown I read was that famous "Da Vinci Code" so I am able only to speak about this one.
I don't think it was written in a poor manner. It does have certain "glu effect" by sticking reader to the book and making him curious about what shall happen next.
But I dislike the book actually.
To start with, I am not fond of thrillers, but even apart from that there are other things.
For example, he keeps speaking about "Da Vinci".
If you take any book by historian of art (or actually anyone who knows something about art), you will never find "Da Vinci", but only "Leonardo".
"Da Vinci" simply says of where he was from (or was supposed to be).
It isn't that I'm meticulous, but this easily reveals a sort of...ignorance?
I am not quality religious educated, but I think we could find more of these "lapsuses" also in religious field, if someone who knows more about it than me would read carefully.
I apprehend that this is a fiction, but I still dislike it.