If I had any private preps around me that offered what many describe on here I would send them like a shot.
I would say that the things described on here are by all means not the norm even for a lot of preps.
Out of the 5 preps we visited, and 2 are well known schools in the region (linked to senior schools with excellent results), none had classes less than 22 children, most being 24-26 with one TA; parents I know told me homework was heavy even in reception and expected to be completed at home (no homework clubs for KS1); after school care available but at a fee; two had no music available until KS2; they did offer things like swimming as part of the curriculum, but I failed to see how a child would learn to swim once a week in a class of 24-26, but maybe they do, not sure.
Also the so called specialist teachers were just class teachers divving up subjects, which to me is entirely different from true specialisms - in a way it is the same in our local primary school, where teachers do lead in a subject for the school, so not a huge difference there.
So be very careful what prep you choose, from my experience of my local geographical area it is very difficult to find one that offers very small class sizes, true specialist teachers and real value for money with all the extras thrown in for free. Also I am a bit cynical about added value on the teaching front when children are loaded with homework from age 4, but maybe I am being unfair. In two out of the 5 I visited I saw evidence of a large number of worksheets/workbooks, whilst in the other 3 I saw evidence of great teaching, with inspirational displays, etc...
But as I said if there was one like others have described on here, I would have no doubts about sending my children to one, it's just that I have not come across one, there must be more prevalent in certain areas of the country. If you look at the other thread, HAPPYGARDENING substantiates a lot of my impressions on here in her own experience and there are a few others who say similar. Just be very very careful and don't be lured in by the gloss, as it is very easy to fall for that. Hope this helps a bit!!!