I have membership in all three (as a Manhattan resident) although I have not yet used the Enterprise one yet. Zipcar is generally good - more expensive than Hertz and has an annual membership fee. I feel like the service and car cleanliness has decreased since Avis bought them out. Hertz was a godsend as competition to Zipcar and their rates are a little lower. As the article says the maintenance and cleaning are worse and I have had cars with fairly major damage and check engine lights on.
As far as my headline goes, though - I thought I had found something very cool when I realized Hertz would let me rent one way to the airport and the first couple of times to EWR and JFK it was great. Then I'm about to leave for an international trip and 15 mins before I get a text saying car is not available. Long story short, no car, no warning and I am scrambling now to find a cab to go to Newark with a shit ton of bags and very little time. Also got ripped off by the cabbie on the way there, but that's just normal. Write that off to bad luck and the exact same thing happens the next two times. I escalate all the way through management who tell me to read the small print - there is no guarantee there will be a car available for a reservation. *Unlike* regular car rental companies (since the Seinfeld bit on rental car reservation) - who now, by NY law, need to honor reservations.
So just be aware of this - if you absolutely need to be somewhere do not rely on these. The previous user may flake out or be stuck in traffic, or Hertz may just change their mind and then you are looking for a cab or car service.
Big bonus for me, though, is that you can accrue Hertz points using Hertz 24/7 as well as regular car rental.