In San Francisco, I attempted to use their service once, and I recall there being a tiny number of cars in the city, and none of them were anywhere near my home.
I used Maven a few times. My biggest complaint was the stupid requirement to unlock the car with Bluetooth. The last time I rented a car, my phone never stayed in sync. I had to reboot my phone everytime I wanted to lock/unlock the car. And you'd better hope your phone doesn't die.