Saw the folks from Urban Safari cruising around SF the other day. They, and their guests, look like they're having a lot of fun. This is one of those services that I wish I would have thought of. Differentiate with a cool vehicle and a personal guide, and you really have something.
Whenever I travel, I always try to take "off the beaten path" guides. You learn so much about the city you are in. I still remember my guide in Budapest, hearing her talk about how Communism really screwed over her parents. I'll never forget that because it was a real experience being relayed to us.
If someone has tried Urban Safari, please leave a comment. I'll try them next time a friend from out of town, is in town.