Polly,
My DS now 6 started pre-k at 2 because he drove me crazy and I thought it would be good stimulation/social interaction for him. He loved it for awhile. The Admin was really good and moved him to other groups to do activities generally considered too old for him. By the time he was 3, he'd tapped their resources dry. I brought him back home at 3 for the most part, kept up with the pre-k as a drop-in on occasion.
It seems like with my son (and alot of others on here) you have to look at things in 1/4's. Something that will stimulate an ND child for a year, gets us about 3 months or 25% of the norm.