We only just bothered with it!! Tried to get Ds1 settled a year ago (he was 3.5) and he was far too unhappy at me leaving him - never really been left before - so we gave up. Went to a diff one after Xmas, it was nicer, he still cried a lot for a few weeks which was horrible and I regret it immensely - i knew it was wrong at the time but pressure was huge from everyone I knew, because I was pregnant and he 'needed' to get used to it apparently
I left him there a couple of mornings a week for a few weeks, then once he was settled, he was Ok just about...but then I had my second child and he didn't want to go back after that, far too upsetting.
We left it till september when he started reception. I was dreading the tears and unhappiness, but actually he didn't cry, he was fine and happy...bizarre. I think I should have left it full stop with preschool, that betrayal might never be repaired, I still feel he doesn't trust that I will not leave him again when he is crying. Worst bloody thing I ever did.
(sorry, rant because it brings back memories!) Don't be pressured - your child might love it, in which case great. But it might not be right for them in which case trust them They will not do any worse at school for missing it!