Exhiliarating! And fun to read as a sort of puzzle, too — one has to do a certain amount of figuring stuff out as one goes along. There was a point late in the story when I hooted with delight at the way a certain plot point (which I had been expecting for some time) was revealed. No more, lest I spoil it for you; take it to the comments if you care.
If you're interested in a taste of the level of philosophy-mathematics-religion-physics nerdiness that underpins Anathem, you can read the acknowledgments on the official book website. If you are interested in a taste of the story, you can browse the book from a link there too.
Me, I'm halfway done, which (as is usual when I read Stephenson) means that I've already read the book through ravenously in a high-speed effort to get to the end and see what happens along the way (because, also as is usual with Stephenson, the journey is worth it even if the end is a bit unsatisfying); and now I need to go back and read at least some of the bits more slowly in order to really understand what I've just been through.
Stephenson fans know that from times to time, story elements and characters (or their descendants) from one book will pop up in another one. I kept waiting for this to happen in Anathem as soon as I began to suspect that it might be possible. I only noticed one significant story element, and I'm still pondering whether it sheds any light on its counterparts in Cryptonomicon/Baroque Cycle.
UPDATE. Oh, hey, I didn't realize people were doing this now, but here's a promotional "trailer" for the book. No, there's no movie, it's just a movie-style trailer for the book. (Jibed surprisingly well with my mental images of stuff, though, especially the sphere, which I have been imagining looked EXACTLY like one of those exercise balls.)
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