It's probably the most detailed and rich description of San Francisco during the Gold Rush that I've seen. The description of the fires and the characters that populate the town are amazing and bring the 1850s to life.
I love that Graysmith has invested so much energy in investigating the relationship between Twain and Sawyer and that part of the book is great. Generally, all the characters in the book are jumping off the pages with all the rich detail given to them. The story of Lillie Coit (nee Hitchcock) is quite remarkable and well told.
The only weakness that stops me from giving it 5 stars is that it could have used a little bit of trimming in its descriptions of the fire engines and the habits of the fire fighters. There was quite some repetition which was not helped by the fact that there are half a dozen major fires in a very short time span - the descriptions of the fire engines start to blur together a bit.
Other than that, if you wan to learn what San Francisco was like during the 1850s, look no further. This is the book to buy!