I'm in France, Gabid, and while I think their system has alot going for it, I'm not entirely convinced it works perfectly either. My DD was champing at the bit to go to school and 'learn properly' and was only happy once she had gone up a year. I think she would have gone mad this year with desperation to learn, if the teacher hadn't put her up. There are another couple of kids in her former class who are desperate to read, but haven't figured it out themselves. (They are asking DD to teach them!)
I think the actual benefit over here is the 4 day week which gives kids a day off from academic work. When my UK friends' kids had crashed out, I was delighted with DD being over here - yes she was bored last year, but she was lovely coming back from school and at home, simply because she wasn't shattered and struggling to cope. She loves school so maybe she would have been okay in the UK - I don't know, but the thought of 5 days a week at just turned 4 scared the hell out of me. Oh and here at 4yrs until 4 1/2 nap time after lunch was still compulsory, although school ended at 4.30.
There are lots of centres for parents who have to work which operate only on the Wednesday / school holidays, and sports mostly take place out of school on Wednesdays. DD's headteacher - infants school from 3-7 - says that she thinks the french system suits the younger / immature children in the year, but is unfair on the older / able kids who end up being bored for at least the last 6 months. She would do more in the year they are 5-6 if she had a say. (She moved DD up a year, but that was because somehow DD had figured out how to read in french.)
I don't think we can cater for everyone, and the system unfortunately is artificial. Maybe a 2 month boundary either way and the parents can decide. It is fairly common for bright Jan / Feb birthdays to be moved up here - and often when they are just 3 or 4 they are put up a year. A teacher can tell if a child can cope or not, and maybe giving them a bit of discretion would be a good thing.
Sorry for such a long ramble.