First of all, this is a long book. A VERY long book. As in, 1130 pages. Every one of which I enjoyed.
So if you're into cryptography, computers, World War II history, submarines, the Philippines, or engineering, you'll love this book. And even if you are only interested in one or two of those things, and know little of the others, you'll still enjoy the story (i.e. I know some WWII history and a little bit about the Phillipines during the Spanish-American War and beyond, but all I know about computers and cryptography is what happened to stick in my mind when people have tried to explain it to me).
The story itself is fast-paced and entertaining, beginning as three separate stories (two taking place during WWII, one during 1999), all of which converge by the end. The three men at the center of each story--Robert Shaftoe, Lawrence Waterhouse, and Randy Waterhouse--are faced with unconventional tasks, including writing and breaking codes, tricking the Axis powers into believing/not believing that certain codes are broken, and unearthing old codes to find a massive treasure buried by the Japanese in the Philippines at the end of the war. Alan Turing (yes, that Alan Turing) plays a central role in the novel, exchanging ideas with Lawrence Waterhouse about how one might build a digital computer. And then there's Enoch Root, the ex-priest who spans the entire timeline of the novel, an enigmatic character who almost acts like a muse or sounding board for the main characters. And even with all of the bloody battles, and long dissertations on cryptographic systems or engineering plans, Stephenson's characters take the time to fall in love with smart, adventurous women.
For me, reading Cryptonomicon was like reading Stephen Hawking's A Brief History of Time or Brian Greene's The Fabric of the Cosmos- for long stretches, I had ABSOLUTELY NO IDEA what the writer was talking about. None. Zip. Nothin'. But the parts that I did understand gave me a general impression of each writer's point, and with enough explaining on their part, I'd start to make some breakthroughs (in Cryptonomicon, this happened for me during the discussion of information theory).
I highly recommend this vast historico-mathematic mammoth of a novel, if only because the characters, for all of their flaws, aspire to noble causes and become better people because of it.