You are lucky. I have had the worst experience with Hertz on Demand. After reserving a car in advance I received no less than five separate notifications each time changing the car and location due to "logistical problems". The final shift was from my original full size reservation to a Fiat 500 at a location 40 minutes away. I cancelled and went to the local rental office where the two people in front of me had the same exact story. Customer service by Hertz On Demand really seems to be a problem in Manhattan. They seem to think they can cancel reservations at their whim.
I can believe one logistical problem, but five times in one week stretches the limits.