It seems to work very well here in Belgium, (high educational standards) but I think the Kindergarten system goes a long way to preparing children for the formal education later.
So pretty much every child has started by their 3rd birthday. They have 3 years of learning by play but in a very structured way.
A lot of the problems I read about on here about the struggle to get used to school don't seem to come up as they are already used to spending the day at school, to using the toilet, getting changed for PE/Swimming, sitting and concentrating on one task for a period of time, getting on with other children etc etc. Tasks get gradually more "educative" in nature. It is a process of gentle transition.
By the time they are 6, and the hard stuff starts, the "social" aspects of being at school have long been covered, meaning the only change they really face is the work!