I had a business trip to San Francisco Monday of this week and decided to take this book along. Turned out to be a great idea - by the time my connecting flight left San Diego headed north, I was on the chapter detailing the San Andreas Fault, which I was coincidentally flying directly up. It's really weird reading abook about the topography and geological history of California while you're flying up the coast at 35,000 feet in clear blue skies on a plane that's only about 10% occupied, so you can run back and forth from side to side as the mood strikes you.
I used the book as a guide to the history of SF on the ground, and learned a lot about the city's history and the significance of certain buildings and places. Oddly only a small part of the book is about the actual earthquake - Winchester has this habit of covering a huge amount of ground as background (some of which is only barely relevant) and if you're interested in the actual details of the effect of the earthquake and fire or how the city was rebuilt, this isn't it. It's more of putting the incident in perspective - not narrating the incident itself. To that extent it was not as useful as his book on Krakatoa, which pretty exhaustively narrated the event itself.
All in all, a good read - but again, if you're looking for the details of the earthquake and fire itself - you'll need to look elsewhere. It would also help if you had a well-illustrated book (or three) on the history of San Francisco handy, because the story would benefit from a lot more visual aids than this book has.