God, I finally finished it. Not having time to read more than about 10 pages most days means it takes a really long time to finish 917 pages.
This book is not science fiction or fantasy. Though it all takes place in the 17th and 18th centuries, it's not a normal historical novel either. Stephenson introduces his own fictional elements into the world in the era of the Thirty Years War, and he goes beyond the sort of details that a straight historical author can make up. Unfortunately, my background in history and geography isn't good enough for me to know for sure how much he made up; I think that the general feel of the world is pretty realistic, but I don't know how many of the characters are real historical figures.
In terms of its style and classification, and also in terms of how it reads, it's a lot like Cryptonomicon. In fact, the clearest thing I can say in this review is if you liked Cryptonomicon you should also like Quicksilver, and if you hated Cryptonomicon, don't even try Quicksilver. If you were indifferent on Cryptonomicon, well, Quicksilver is almost as long, and it's only the first book of a trilogy, and although Quicksilver was engaging and entertaining, it didn't suck me in the way Cryptonomicon did. Finally, if you haven't read Cryptonomicon, I do recommend it highly, and suggest that you read it first.
I did enjoy reading this, but it was awfully long. I think I'll give it an 8 out of 10.