When you factor in gas, Zipcar can easily be cheaper. If you live in SF, for instance, Zipcar is most likely the cheapest way to rent a car to go to Tahoe. On a trip last year my (Zip)car beat the other car by about $20 after all was said and done. So the $75/day can be misleading at times.
Also, unrelated, Zipcar allows members who are under 25, which makes it much more expensive to rent a car from a traditional place like Enterprise. Being currently under 25, this is a massive benefit.