The book is primarily useful for someone designing their own cities, and people using Waterdeep, Sharn or another published setting won't have as much use for much of the material. However if you're the kind of GM who makes the (published) city your own by adding additional guilds, organizations, and the like, you will get some mileage out of it.
If you're not looking to do much creating, there's some feats, spells, and new monsters, a bit of general advice on running urban games as well as some nice pre-statted urban NPCs.