DD started preschool/daycare last year a little while after she turned 3. Initially I wasn't sure it was a great idea but she's so social and loved it so much when we went to visit I figured it would be good for her. The first few months were great, then I guess the excitement wore off and she started crying when we took her in to drop her off. I think part of it had to do with dropping her off early before most of the kids and other teachers showed up. I did keep her home some days because I felt bad, but sometimes I had no choice but to take her and when I picked her up she always had exciting stories to tell about the day and sometimes she didn't want to leave. When I started working part time later in the year she cried a couple times but DH said that she would immediately get distracted with something else.
She stopped school in December and had been practically begging to go back so we signed her up at an actual preschool for fall. Now that it has been a year the experience is much different. She's always excited to go to school and never worries about us leaving. The other kids cry though and it's really hard to watch. I don't think I would want to take her if we had to go through that - it's only for a few hours anyway and it doesn't seem worth it to me to have to go through so much separation anxiety.