I signed up and have been using Hertz on Demand for about 2 months and so far have had a good experience. They do have less locations it seems than Zipcar, but there are many close to my apartment, so it works out fine.
Their cars seem very new which is also nice. They have always been very clean inside and outside.
I'd also suggest adding them as a page on Facebook (search for Hertz On Demand and Hertz)- they often run weekly specials on a particular version of car at certain garages, as well as a monthly special that applies to a particular car type at any location. For example, last month the Jeep Liberty was on special for 7.50 an hour (50% off).
I believe they're still offering $50 in credit if you sign up too - it will look like they will actually bill you when you rent a car because they will authorize the rental amount to your credit card, but after you return the car the charge will drop after a couple days.
I agree the online interface kinda sucks, but overall its been a good experience.