You have a lot of good resposes, just thought I would throw in my experience. It is said that kids do beneift from it which I beleive can be true but I have an older son who went to preschool and doesn't do as well in school as his younger sister who is at the top of her class.
Right now money is an issue for a lot of people so I found ways to help with the pre-school experience with out the extra expense. Most libraries have a toddler reading time which they put on at no cost. Plus there are a lot of different classes for kids at Public Rec groups which can help kids socialize. Also at home there is so much we can do that they do at pre-school. Music time, craft time, a lot on the web were we can get ideas.
Hope this helpped a little.