These guys are very reliable, if expensive, way to get around Manhatten. John Connors recommends them and we use them in the city. Very on time. It is actually run by an ex-cop see http://www.usnews.com/usnews/biztech/articles/040607/7eebody.htm for his story.
It is kind of a kick to get picked up by someone who is as big as a house and whose car has a siren. Very handy for getting through traffic.
They do all kinds of security details, but they also will just take you around town, if you absolutely positively have to get there :_)