Food Co-op's - How Do They Work, Good/Bad Experiences?

I've heard of people here and there mentioning that they are in some sort of food co-ops, but I've never really personally looked into them much. I've tried looking for some information on ones in my neck of the woods, but the sites themselves don't seem to provide much details on the programs.

From what I can tell so far, you basically purchase a few months worth of produce in advance for a couple hundred dollars (about 3 months worth) and I guess pick up your stuff weekly. I guess that's how they work around here, but the sites don't explain much more.

Is anyone else on here in one of these programs? If so, was it worth it to you? Did you end up saving money? Was the quality and selection good? I'm interested in anyone else's experience with these.