I've been a member for over 5 years. I live in a popular neighborhood in SF. It's to be expected that weekends or special events affect availability. But there are so many lots around its easy to walk an extra block.
I've been in 2 accidents in their cars. First time was pretty bad, 2nd one was minor. Each time the most I had to pay was a $500 deductible. That sucked, but was nothing compared to how much I had to pay for an accident many years ago in my own car. (This all makes me sound like a terrifyingly bad driver, but I'm not!)
Just in the last 2 days I used Zipcar. I was heading to Stinson Beach for New Years. To rent the car overnight was $101. It was 2 blocks away.
I called Enterprise downtown and got quoted a car at $28/day. Awesome, I think. Well, no. They closed at 3pm that day. And they're downtown. And although I only needed the car for 1 day, they were closed on Jan 1 and would have cost me $45 to drop the car off when the place is closed. So then I would have bad to worry about parking a car I didn't need. Also, since I refuse to carry a credit card, in order to use my standard bank card (the thing that has all the money I actually have, and not artificial debt like in a CC), they want paper copies of two bills to my house and my last pay stub. Seriously? I don't get paper anything anymore, much less a pay stub.
So in the end, I payed a little more for the Zipcar rental. I got the convenience of it being local, flexible, and less shit for me to worry about. That's why I like Zipcar.
I've had the occasional frustration with them. Dirty car, no gas, broken card. Little infrequent things that are frustrating at the time but are rare. You call and report the previous driver, they get fined and don't so it again. It's happened to me a couple times.
So I refute your claims about Zipcar being so bad. I've used them a lot longer and quite happily too. I'll continue as long as they don't become like other rental car companies.