My son uses ZipCar, he lives in Somerville/Boston. He started using it when he was in college at UNC. Works great for him, I believe he said he averages about 6 uses a year. I will be interested to see how well the Enterprise version does. They have rental agencies in a lot of areas so they already have the footprint.
I don't think the price is outrageous either, especially if you plan ahead to maximize your time.
I just got the tax/tag bill for my personal truck and it is $100 for the year, plus inspection will add another $15 to that, it would be $45 by my truck is exempt from the OBD/emissions. Insurance is another $360
We drove it less than 3500 miles last year. If I lived in town I probably wouldn't have it anymore.
I just looked at the website, looks like a decent deal, time will tell if the car will be available when you need it.
Aaron