I read this book in about 4 days - I think it was a great page turner and nice escapism. I agree with many of the other comments. Main thing I thought was that it was really formulaic, and that started to drag fairly quickly. It was obviously written for a film - Speed meets any John Grisham story.
There were some EXCELLENT cliches - loved the psychotic albino monk,

and the name of the 'arch-baddie' - the TEACHER (wooooooo)8-[ ! I also guessed a lot of the 'codes' - the mirror writing one, DOH!! Will anyone own up to not getting it? I'll admit that I didn't get who the teacher was though.
I also enjoyed all the religious conspiriacy bits - although for something that is supposedly so SECRET there are, and have been for a long time, a lot of people discussing these questions. It doesn't seemed to have weakened the strength of the Vatican so far...
So yes, I'd agree, I can't really understand why this is such a massive seller. But I'd recommend it for light relief.