I'm about a quarter of the way in, and it's a very good read. Many familiar surnames from Cryptonomicon reappear, and at least one familiar character. But much of the action is set in and around seventeenth century London, which makes it all seem much more personal -- I buy my sandwiches for lunch where he's placed Comstock House, and tomorrow night's anonymousclaire meeting is just down the road from the Waterhouses' London residence.
The principal character is Daniel Waterhouse, presumably a remote ancestor or greatn-uncle of Lawrence and Randy Waterhouse from Cryptonomicon -- and he's similar to them in many ways. A bit unworldly, and hanging around with the great minds of his time -- where Lawrence struck up a friendship with Alan Turing, Daniel has Isaac Newton.
Coincidentally, Ken MacLeod recently posted (on his excellent blog) one of the best definitions of SF that I've seen, which (loosely paraphrased) goes roughly as follows: For thousands of years, people huddled around campfires and told stories. Some of the stories were about the people around the campfires, and some were about the gods, ghouls and ghosts that they saw in their distorted, flickering shadows. Then, some time in the seventeenth century, the sun came up, and science fiction is what we call the stories they tell about the landscape they can now see all around them.
Quicksilver is the story of the sun coming up, and the landscape beginning to become visible, stretching out all around the campfire for as far as the eye can see.