DD did enjoy it, there is no bullying, etc that may happen in a bad state school. And the teaching was fine. But I would have to say, that was that. The price we paid for it was not really worth it, either in academics or otherwise. It is a good school, but only as good as most good state primaries.
At the secondary level, it is indeed brilliant. I would not recommend anyone spending that sort of money before that, especially if it is hard earned. We did it for a short while only till we could get a place at a decent state school, and then moved out.
While we were there, we had high expectations for the price we paid, but we found very little difference at the prep level between the 2 good state schools where she studied, and SH prep. I am sure it makes a difference at secondary school level, but not in lower years.
I can confidently say all this since DD is now in a top grammer school, doing us all proud, aiming at bigger things. And she got there while doing year 5 and 6 at a decent state primary.
For me, DD's education came before everything else, so, I decided to spent the extra money while we were between 2 state primaries, in between our moves. But if I had a choice, I would have saved that money and used it for some better cause for her own future.
Having said all this, if you have rather unlimited funding, or a big inheritance, then why not, give it to her as a gift for her to enjoy, but dont do it thinking it would make any difference to her life.
If it is your hard earned money, consider investing in SH at secondary level, or maybe 1 year before that, just to be sure.
Hope that helps.