We did it with DD1 how is now 13 (yr9). It was difficult decision, as she had been in an excellent selective school on 7+, and would have automatically entered seniors.
Our reasons were mainly financial, bu partly made easier by her being unhappy there since mid yr5. Basically teachers were pushing her harder than she was ready for. She was always well within the standards for the school, but they expected that bit more which she either couldn't or wouldn't give.
She got into a reasonable comp. and there were about 10 or 11 from private schools, although she was the only one from a selective school.
She was well ahead (possibly as much as 2 years in some subjects) when she started, and up to now has got by with doing very little work. she has had extension and G&T work in some sujects, but in many she has been allowed to coast. Although she is still on higher than average grades for yr9 (except for English which is another story completely) and in top sets for everything setted, we worry now that as the work becomes more difficult getting towards GCSE, she will have forgotten how to learn.
Bullying was a bit of an issue in year 7, and her accent dropped to some extent. it also took her a while to get used to being taught alongside kids that did not want to learn, and hearing swearing in the playground that teachers largely ignored.
I will always have mixed feelings on the move though. Without a doubt the secondary education would have been better at the private school. She would have had the opportunity to do Latin/classics, no compulsory ICT GCSE and wouldn't have been forced to do BTEC sport on top of 10+ GCSEs. But I am not sure she could have took the pressure, and being an August born didn't help. She has recently become more aware that she is getting an inferior education to what could have been, and would like to go back to private for 6th form - but that renains to be seen.
On hindsight, it would probably have worked out better if we had not sent her to private primaries. Our two local primaries are excellent. But as she was in nursery of a prep school and we were not expecting to have any more children, it was easier to continue that route. DD2 (8) is in yr3 at our second choice of state primary and is flying.