I've ran a Ptolus campaign for the last 2 years, just off the information on Monte's website. When I was coming up with my homebrew I realized that Ptolus already had a lot of ideas that I was intrested in: A Warhammer like dark automosphere, a Chaos/Cthulu bad guys, lots of room for political intrigue, a aging failing Byzantine like Empire, a large central church.
Since Monte is much better at naming things then I am, why reinvent the wheel.
It was a great decision. The Organizations I think are the wonderous thing about the setting. I knew I had stumbled onto a blessed thing when my players had a 30 minute in character discussion regarding what to tell their Lothian Ally regarding an encounter they had with Helmut Istlestein, the leader of the Ptolian independece movement.
Playing in a city is great. Often times the enemies have power beyond stats, and the groups enemies and friends are close. The illusion of player choice is easy to maintain, especially when you can count on droping a clue, and reasonably expect the players to come up with going to see their Allies to flesh out the needed information.
I look forward to the book, even though I know already my campaign from 2 years of playing, will probably deviate from the book by quite a bit. Banewarrens and the Chaositech alone are almost worth getting it. Given the Night of Dissolution adventure, and all the adventure seeds in the book, I expect 3 full campaigns can arise pretty easily out of it.