When my ds8 was of preschool age I was working and he was in a daycare center that also served at a preschool. However, when he started Kindergarten I quickly learned that the "rules" of preschool were not as strict as they were at school. He didn't like sitting quietly and still, raising his hand, waiting to speak, etc. He also had trouble socially because he was used to a more relaxed environment. His teacher finally got the point across on how he was expected to behave but it really did take 2/3 of the year.
Dd6 was taken out of that same daycare when she was 3 and was kept by my older dd (along with my ds). She attended 1 year of preschool when she was 4. It was a Christian preschool and they were as structured as the public school she now goes to. We never had any problem with her transition into the school system - she has the same teacher that ds had.
I sent her to preschool because I was worried about her academically - she had no desire to learn anything - no letters, sounds, shapes, simple words, etc. Ds learned a lot from The Letter Factory and The Word Factory and other similar DVD's but dd was not interested in watching TV either. Within a couple of weeks of being in a school environment, she took off and by the semester break she had met the requirements that they would be teaching the entire year. She went into Kindergarten this year very prepared and has been reading quite well for the past few months - I don't think she would have "gotten" it quite that quick without the preschool. If I could only reverse time and put ds in preschool, but we're a few years late for that now since he's in 3rd grade now.