My dd went to the local 'outstanding' state school and we moved her at the beginning of Yr 2. It was a very hard decision to make, but the right one for her. Large classes, some 'challenging' characters (the HT's description not mine) and generally a feel that she wasn't really achieving and being 'herself'.
I have to say that I am staggered by the education she is getting now, it is a million miles away from what she had. Obviously this will very much depend on the standard of the state schools and the equality of the private school, but in our case it shocks me there is so much difference (I was state educated in a pretty rough comp.).
Our dd is now the bubbly confident child she was when she started school in Reception, but then we lost part way through and at the beginning of Yr1, she has settled in well and everyone is amazed at the change.
You have to do what is right for your child and your family, although you won't know really until they start school what is going to be right for them!
I am glad in someways she went to the local school, she has made a lot of friends in the village and we now won't be thinking 'what if' if we had sent her straight to the private!
However, she has had some catching up to do, particularly in maths and her handwriting, and I do wonder how different things would have been if she had gone there from the start.
Rabbitstew I feel you initial post was unfair, the OP asked a honest and difficult question - Seeker, we have heard it all before!!!