What It Is: ZipCar is a car sharing service, which lets you rent (or share, if you prefer) a variety of cars by the hour for a low fee that includes everything — even gas. ZipCar recently acquired Flexcar, so they’ve got cars in a good number of cities, and I’ve been able to live in both San Francisco and New York without owning a car at all, thanks to mass transit and ZipCar.

The Experience: ZipcCar’s got a great website and a really easy signup process. You put down yer credit card and some ID and choose a plan. The bigger plans are good if you use the cars frequently, but even the most expensive plans are so much cheaper than owning a car it’s ridiculous.

The cars themselves are great, too. Though I lament the fact that they’ve phased out XM radio in the vehicles, the amenities are great, with the higher-end cars having gadgets like GPS and all the cars being zippy and fun-to-drive.

We tend to get the little errand-running cheap cars, like a Toyota Matrix, or even a Scion, and those work perfect for stuff like taking the dog to the vet or visiting big box stores in the suburbs once in a while. On my birthday last year, we got a convertible Mini Cooper to zip around the coast in Marin, and I can see getting a BMW or something if it were a special occasion. (Fancier cars cost a little more, naturally.) One amusing bit of branding is that the more you pay for the car, the less obtrusive the ZipCar branding is. Cheap econoboxes have big ole’ ZipCar badges on the side, but the BMWs only have a discreet little sticker on the bumper.

ZipCar ends up being a lot cheaper than regular car rental for any duration up to a day, especially if you factor in gas. Buying gas with a ZipCar is a simple matter of using a special payment card that’s kept on the visor of your car. Even getting into the cars is easy — the ZipCard you get with your account just gets swiped over a card reader on the dashboard and the locks pop open.

In the cities where I’ve gotten ZipCars, there are lots of parking lots with many cars available. The variety and number of cars probably varies a lot by city, though. I’ve also found that if I want a particular style of car, especially during a busy time, I might have to travel a little further to get the one I want, but it’s still just a matter of a few blocks. To get a truck or van for a trip to Ikea, I might have to hop on the subway or go across town, but that’s no big deal. And I never feel too worried about having to have a car back by a certain time — if there’s no reservation after yours, you can call and extend your reservation on the phone really easily.

ZipCars are even cheaper than a taxi for a lot of options; When we lived in San Francisco, a cab from the airport to home was easily $40. But getting a pretty nice ZipCar for the jaunt down to SFO and back home was about $12, with no tip required.

And of course, there’s the green factor. It’s unquestionably satisfying to not own a car and not have to be responsible for all the attendant hassles. But the truth is, having not owned a car for a decade now, the idea of buying literally tons of metal and glass and plastic so that it can sit idle most of the time, rapidly losing value and only being of use when it’s burning increasingly costly fuel just seems like some kind of punishment. Granted, it’s an important and practical consideration outside of urban areas, but in a city with a world-class mass transit system like New York’s, having a car would just be an unjustifiable indulgence for the way I live.

The Gotchas: Of course, ZipCars aren’t perfect. If it’s a weekend or some other busy time, you might have trouble finding the car you want at the time you want. If you’re looking to grab a car for a long block of time, like a weekend, it can be frustrating if someone’s booked your favorite car for one hour right in the middle of that time.

And of course, the cars are shared. I haven’t encountered a dirty or damaged car, but I’m sure that could be an issue for people. There certainly are times that the gas tank was a little on the empty side and I had to make an annoying stop at a gas station at the beginning of a trip. But the one time that a car I had reserved was being returned late by the person who had it, ZipCar actually called me proactively, let me know about the issue, automatically moved me to a reservation in the next nearest lot, and upgraded me to a slightly fancier car for no charge.

What It Costs: Cars vary in cost, but the ones we book most of the time are less than $10 an hour, and having a car to use whenever we want cost less than $600 last year, including gas and insurance. The basic application fee starts at $25.

Recommended If You Like: Changing cars to suit your mood, using mass transit, the Earth, spontaneous little road trips

This post is one of a series of unsolicited testimonials. Please view that introductory post for more background information.