Vornheim: The Complete City Kit is, without a doubt, one of the most useful RPG books I've ever come across. If you plac to run adventures in cities--or allow your players to have their characters run rampant in cities--you owe it to yourself to purchase this book.
While it has a fair amount of information on the eponymous city, if you don't plan to run in your version of Vornheim, many of the snippets of information are great for inspiration and kicking off ideas for your own cities.
The rules for urban crawls and randomly generating neighborhoods and houses are wonderful.
The charts and tables--including ones you are intended to print out and roll directly on--are extremely useful.
I really can't say enough about how great this book is, and the PDF is worth every bit of the price.
[5 of 5 Stars!]