I read The Da Vinci Code a few years ago and wasn't very impressed. It read like a cheap action film to me. An easy fun read, but very over hyped. I wasn't in any rush to read any more of his books.
Imaging my surprise, if you will, when I started to read Angels and Demons and got totally engrossed. It has some of the same awkward writing that the other book has, and also reads like an action film script, but the plot, atmosphere and characters are superb with plenty of clever twists throughout.
The book is mostly set in modern day Rome, and I actually visited nearly all of the locations in the city on our holiday there last February. Knowing the places first hand really helped with the sense of atmosphere for me.
I could nit pick the particle physics near the start of the book, but other than that it is surprisingly excellent. I love the ambigrams.
I read his book about encryption (Digital Fortress) and found it completely awful. Complete lack of knowledge about his subject matter, and a really silly plot, avoid like the plague.
Yeah, Rob D told me Digital Fortress was terrible.
Angels and Demons is probably Dan's best to be honest. I haven't read too many, but that's the only stand-out. The Da Vinci Code sucked :)
A good book that's (partly) about Cryptology is Neal Stephenson's Cryptonomicon. A really good book written by someone who knows what they're talking about.
Although it was written before the Baroque Trilogy (QuickSilver, The Confusion, The System of the World), it's a sequel of sorts since the main characters in Cryptonomicon are the descendants of the characters in the trilogy and it covers some similar themes.