I sent my daughter to preschool twice a week this past year. It was my husband's idea and I was not too thrilled with it, but then since I was going to be having a baby, I thought maybe it might be a good idea. The first month was very hard. She was the type of child who didn't like to be in another room of the house from me, and she always wanted me to stay at the school with her. The first day she was fine, but it got worse from there, and she would state very firmly on Tuesdays and Thursdays that she didn't want to go to school. But she would get ready and get in the car even so, and I was usually able to convince her she didn't completely want to stay home because there was something going on (field trip, pizza, show and tell). I told her that if at the end of a month she still didn't want to go, we'd quit. By the end of the month, she had a good friend she always looked forward to seeing, and she would run in and barely look back. So she did get used to it, but it took a few weeks. Now she loves it and gets upset if for some reason she can't go.
My personal feeling from things I've heard from others is that being ready to go to school is more a developmental thing and going to preschool really isn't necessary. I've heard several stories of children who had never been away from their moms, but went right into class on the first day of kindergarten with no problems. I also heard a story about a little boy who hated preschool and never wanted to go the entire time he was there, but he loved kindergarten and couldn't wait to go. That was someone my sister knew, and she has 6 children herself. Only the first 2 went to preschool, and they are the two that ended up dropping out of high school. The other 4 had no problems starting kindergarten when they were ready (5, almost 6), and are all doing well in school. So I really don't think preschool is necessary, although in my daughter's case she really likes it and I think it has benefitted her. She wants to be doing stuff and she gets to play with other children and do crafts, more than I can provide for her to do, I guess.