Wow. This was a LONG book. And I did it on audiobook. It was, what, 27 CDs, I think. And, at the end of all that, I'm still uncertain how I feel about it. Without a doubt, I loved parts of it. The whole idea of mathematical monks living in a monastery, the concept of layers of monks, some of whom can only see the outside world once a year, some every ten years, some every hundred years, and some every thousand, that was all cool and fun. Really, what I enjoyed most was hearing the traditions of the monks, and why they were that way. One that really gave me pause was a tradition where every junior monk was assigned to a senior monk, and was to attend to their needs at meals. The idea being that this gave the juniors a chance to overhear and discuss (in the kitchen) what the elders were talking about. I often think about how I wish my students were able to overhear faculty conversations in this way.
So... for all the cool stuff I liked, there was maybe an equal part that made me roll my eyes. Action scenes in the arctic, in space, basically, all the action scenes I could have done without. And Neal Stephenson has this other habit that drives me crazy -- he goes into great detail about some small thing that isn't very interesting in itself, but also plays no role in the overall story whatever... and I look back thinking, "why in the world did you spend time talking about that?!" Historical battle recreation with weeds, I'm looking at you!
Anyway... this was thoughtful, imaginative, and fun, with lots of interesting surprises. It's all told first person, too, which makes for an interesting novel - more interesting, in some ways. When it was over, I was kind of sad - because I liked the world he crafted, and I realized there'd be no going back. In the end, I was really surprised to realize what a solid, compelling world he had created.