I am going to keep this short, but this is probably the best city supplement ever released. There is no other city like it. I ran a Planescape campaign setting for ten years and with the whole multiverse at my fingertips we still rarely left Sigil. Why bother to explore the multiverse when you can stay where you are and have the multiverse come to you?
As another reviewer mentioned in addition to the city description there is are a lot of sample handbills, menus, advertisements and official proclamations that provide a great amount of flavor and make great player handouts.
[5 of 5 Stars!]