I think it totally depends on the child. I enrolled ds when he was 2 1/2 in a coop preschool (once per week and I was with him the whole time). I can't say he really got much out of it. Though it was nice to have playdough, painting projects and other messy stuff that I didn't have to clean up (unless it was my turn! LOL).
My dd went at the same age and loved it. She is much more social and has fun with lots of kids around. She went half days M-F, and her big brother was in class with her (mixed age preschool class).
If I had it to do over again, I probably would cut out a lot of activities we did when the kids were really little (gymboree, music together, etc.) and put them in a savings account for now when there are activities that they want to do.

I think they had enough fun experiences with free playdates and activities at home before age 4 or so.