Your zipcard works with zipcars nationwide, the rates just might be a little different. But you can reserve any of them in any city. That's actually great. I've had zipcar since living in SF before law school, maintained membership in law school (Boston), spent a summer in DC, moved back to SF. I actually still pay my annual fee even though I now own a car just in case I'm in some city where I might want it. I kept my zipcar membership despite having regular access to a car for the second half of law school because I liked having access to a broad range of vehicles for moving, etc. In theory, the annual fee varies from city to city, so you could choose which one you want to be your home base, but I haven't looked into it.

Customer service can be a bit slow for stuff that is out of the ordinary. I got a parking ticket in a zipcar, which I paid, but zipcar billed it to my separately and it took forever to get it resolved (though it did get resolved).

Oh, re: time limit, there is no time limit, but you pay hourly (with a cap at 8 hours per 24 hour period, I think), and if you needed it for a long block, you would probably need to reserve far in advance. (Note that there is a penalty for late cancellation).