What can I expect out of a Critical Mass experience?

Critical Mass.

I’m thinking about riding in my first one tomorrow night. Got my bike all ready (no, not the beautiful Gypsy, but a beginner bike) but I’m not sure what to expect.

Do any of you ride in your city? What’s it like?

Also, I may end up going alone, so is this the kind of thing that’s ok to do alone? Or do you need the buddy system?