We love our Montessori preschool. Our boy started when he turned three, and he refused to just stay for half days as he didn't want to miss out on anything. So he's there from 8.30am-3.30pm two days a week, and he'll do an extra day when he turns four. He is also a quiet/shy type, but has made lots of friends and is very happy there. The only downside of the structured day for DS is that he gets pretty tired by home time. Some parents of very energetic kids find there are too many indoor activities and not enough time spent running around wildly outside - I know of one parent who switched her four year old to regular kindy for that reason.