I'll split the bounty with you if you join! (That's $25 for you and $25 for me, not $12.50 each...)
As a dedicated walker/transit user I don't use Zipcar very often, but I love it when I do, and I love having the option to use it at any time. If it's available in your city (or even a city you visit often, if transit doesn't meet your needs there) you should definitely check it out!