My second favourite Neal Stephenson novel; just beaten by
When this first came out, it was partly set in the present day and partly during the United States' involvement in World War 2 — so now, of course, it's partly a historical novel set in 1999 (pagers! GSM phones! BeOS!), involving a bunch of cypherpunks attempting to set up a slightly-shady digital currency. Which has, of course, happened in the meantime.
In general, this has aged pretty well. It's structurally very neat, being written as an ordered set of standalone vignettes (many of which would make decent short stories) rather than as continuous narrative. It's a long work but not an overpadded one, and maintains an even pace throughout. There's certainly enough complexity here to reward rereading several times, especially if you've gone away and read more of the relevant history in the meantime…
(The typesetting in this paperback edition leaves quite a bit to be desired, though, especially the maths near the start and anything set in a monospace font. It's not unreadable, but it is jarring. Other editions may be better; I've certainly not had the same problem with Stephenson's more recent books.)