I love co-op preschool! But, I am very *picky* about what I call a co-op. For example, the Y runs a co-op near us but the parents only volunteer once a month. That doesn't do it for me.
In Eugene, OR we got hooked and now go to a great co-op in WA. Anyway, DD goes 2 days a week (she is in the threes class) and DH or I go one of those days (we go Thursdays). That way there is a 2kids:1adult ration plus the teacher. Each parent has a "room" for the day (art, blocks, play, story, kitchen, sand, woodworking) and then extra tasks (either cleaning up or supervising out door play, etc...). We have a class meeting once a month in the eve and around 3 special speakers per year (this year we have had authors on "tantrum taming" and "positive discipline."). Next year, in the fours, we (DH &I) will still go one day but DD will go three days per week. It is a great way to slowly transition away from each other in prep for "real school." DD's best friend goes to co-op in the neighboring town and they to 2 days for 2 year olds, 3 for 3 year olds and 4 for 4 year olds with parents coming once a week still.
I really like it because we were looking for a totally non-educational (well, not stressed) environment. We wanted a lot of social time, especially with other adults. DD is great with other kids but has always hung back around other adults. She is much more outgoing around adults now, and boy is she friendly with kids. We also didn't want her bored and were afriad that a more traditional preschool would be not on her academic level (since she can read, count infanintly, etc...).
Another advantage is instead of like "Art time" there is activity time where the kids can play anywhere (since there are adults everywhere). So if DD doesn't want to do art at 12:15 she doesn't have to, but there will still be the opportunity later! At some preschools with lower adult/child ratio the kids have to follow more of a schedule since there are only 1-2 adults to help them (so they have to do art when its time).
I also like that basically all the kids have a stay at home parent (since they have to be available during the day to go to preschool). We also are very "same wavelength" (most of the parents don't leave their kids with sitters often, really respect their rights/choices, etc...).
Anyway, you just have to find what works for you!
HTH, sorry for the novel,