The decisions we have made personally for our children are this -
We feel very strongly that private education is superior to the state provision we have locally. Two of our five children are currently in an independent senior school and we are very happy with the school
I have worked in many schools - good, bad and ugly. I feel that our choices are more informed than most. My experiences include an Outstanding comp, and a grammar school - but these do not sway my opinion one iota.
My three girls are in a state primary - middle of council estate, added value below 100%. Each of them is at the top of their class. I work in a girls' independent school, and my plan was to move all three to my school ASAP. However, the finances didn't work (they never do). I would be prepared to struggle to send them, but DH was not willing to go into debt. His opinion is that private schools at primary level are for rich people, and that is not us.
I would love to have all three of my girls at my school, but it is just not possible. DD1 will join me next year as she moves into Y7. My other two will both come when DD2 moves into Y7. Saying that, I am content with the education my girls are getting in their primary school. The SEC profile of the school is not good, but they do a good job of protecting the vulnerable pupils from their situations (basically poor parenting). I despair for these pupils when they moved to secondary where they will not have the very loving pastoral care.
Obviously it depends on the specific primary school, but we have found that our children have done well academically. Socially has been a problem. The lack of cultural diversity has been difficult for DD1 - she is American and no one really clicks with her. She will be much more comfortable when she moves to my school because there will be several other oddballs in her class.
If you are worried about how your clever child copes in a state primary - I wouldn't. We have found that clever children do well enough. Only go private at this stage if you can truly afford it. It's a different story at senior level.