We've been very happy with the experience that our 3-year-old son has had at Morning Star Preschool. We chose it primarily for the social structure that it provides, and have been very pleased with the results. Miss Karen and her staff provide a very structured, consistent, and effective atmosphere for the kids, as well as very good feedback and advice to us as parents on how we can reinforce good behavior, react to bad behavior, and good answers to questions that we have about any kind of developmental issues.
They told us up front that if we were looking for a rigorous academic program that we should look elsewhere, but since we have no concerns in that area we didn't worry about it. (He's naturally very curious and smart, so doesn't need any special encouragement there - his favorite magazine is Science News, and simplistic explanations of the pictures is NOT tolerated - trying to explain a diagram of quantum tunneling exercises my mind as much as his, I suspect...). Despite that, from what I've seen they do provide appropriately challenging activities - they just don't force the kids to progress at any specific rate.