I don't usually place much weight into the book reviews publishers tag on back covers, but the review on this book really does describe it - this book is Tom Clancy mated with William Gibson with James Mitchner acting as a midwife.
Even though this book is an astounding 1200+ pages, it is an engrossing read. Like Mitchner, the story weaves the lives of many generations together through a common theme. Except Mitchner never wrote about lives so exciting (Apologies to any Mitchner fans - but Hawaii was a little dull.)
There are many character threads and stories in the book, but the two main ones are the story of a WWII cryptographer (Clancy style), and the story of his Silicon Valley grandson's pursuit of an offshore data center and advanced cryptography (Gibson style). Both threads are thoroughly engrossing. The book paces perfectly, it never gets too frentic or too dull.
The character development is also done very well - Stevenson doesn't clutter the book with too many marginal characters besides his main ones and he makes most the characters very memorable. This leaves him lots of time to develop his main characters into complex and interesting people.
Stevenson's writing style is also very readable, yet not as flat as the standard supermarket fiction (or bad sci-fi for that matter). The different story threads are written in a different tone, and Stevenson uses his command of tone to provide even more character and plot development. For example, his savant WWI cryptographer thinks in mathmatical proofs, his modern-day cyberpunk in Tolkein-inspired metaphors.
If I had a complaint about this book (I don't have many) it is that the ending leaves a little to be desired. I won't give anything away, but my overall impression with the last 200 or so pages of the book was that Stevenson got tired and just started typing out some text to finish the thing up. It's not a complete breakdown, but compared to the rest of the book it is a weak showing.
Regardless, I still highly recommend this book to any cyberpunk fans, war story fans, or math geeks.