Dan Brown is an undeniably clever guy. He put together a mix of religion and esoterica into a serviceable thriller plot and got one of history's most widely read novels. He tries to reproduce the phenomenon in his new book, using patriotism instead of religion, and fails to get the mix right.
For one thing, there is way too much esoterica. Long patches of dialogue read like Wikipedia texts, turning the novel into an encyclopedia of arcane knowledge. Layer on a Washington, DC, edition of Trivial Pursuit and you've got mystery nowhere near as potent as Jesus establishing a royal bloodline.
The weakness of Brown's protagonist, Robert Langdon, becomes glaringly apparent in the absence of a riveting plot. A symbolist who remains a cipher, Langdon is almost totally devoid of personality, and is not even a two-dimensional character. It's appropriate that the bland Tom Hanks plays him in the films. Once again he has a female helper, but once again Brown shies away from anything like full-throated romance.
Brown's compression technique -- having the action take place in a 24-hour period -- has been intensified to a 12-hour period, that defies credibility.
Personally, I like a lot of the arcane stuff, but this book is top-heavy with it and often repetitive. It is of course at the top of the bestseller list from pre-orders alone, and Brown has created a franchise that will survive this book and go on to another bestseller. But this is no Da Vinci Code and is really only mildly entertaining as a thriller.