The reason I keep coming back to Zipcar is simple -- they offer a great product at a great price and have great customer service every time I need to contact them. This summer my bf and I got a 6 game ticket package to see the Phillies in Philadelphia. We used Zipcar for every trip -- picked up the car at a garage 2 blocks from our house, got to drive high-quality well kept cars (Honnda Civics), and dropped back off at the garage with no hassle.
In addition to the great customer service, Zipcar is also hassle-free. They pay for gas and tolls!