I was so focused on sharing pictures and narration of this year's art co-op that I neglected to mention some of my favorite features of co-ops in general: shared effort and accountability.
Co-ops are useful for sharing planning or execution (or both) of subjects that are time consuming or in which one mom has more skills than the other(s). If one is good at science and another at writing, maybe you can get together once or twice a week to cover both subjects.
Our preschool and KONOS co-ops involved academics and activities, and required lots of effort, but because many families were involved, most weeks were easy for most moms.
Accountability is probably the most important aspect of co-oping for me. That Henry Moore plaster project? For "just" my kids? Probably not happening. I make a better, more consistent effort when I know I'm responsible for educating other children, too. Co-oping is a way for me to follow through on doing what I WANT to do, especially with the more hands-on side of school.