I started my daughter in pre-school last month, when she was 26 mos old. I felt it was time because she speaks very well, already knew her colors and counting to 10, and just seemed to soak in anything I could teach her--I felt she needed the additional enrichment of pre-school. As my first child, she also would benefit from regular time with other kids, and with other adults as authority figures. I'm blessed to be able to work from home, but having her in school 1.5 days/week also would allow me to work in the office a little, which was a nice benefit (it's very close to her school).
My daughter is outgoing and happy but also very sensitive, and though she liked school from the get go, she had a hard time with me leaving her anywhere. At the beginning I'd have to carry her in crying, and they'd peel her off of me as I made a quick exit (the faster the better for the child), and when I picked her up they would assure me she stopped crying very quickly as she got into playing with toys and the other kids. A little over a month into it, she walks in on her own power, but still cries when I kiss her goodbye...though she immediately goes to the teacher to hold her, rather than clinging to me. They tell me she's doing great, and they're ready to move her up a class because she's mastered everything in the toddler class! She also loves to tell me about what she did at school that day, all about her teacher (who she loves), and I'm starting to learn things about her classmates too.
I really notice some changes in her, with learning new things, being less shy around new people, and overall having more confidence in herself. I have no regrets even though the morning tears are sometimes tough. I think she'll be over that in another month, though. She really does love it, but she loves her mommy, too!
Good luck!