I know this is an old thread but:
I worked for PGL in 08 and have to say I think the kids bloody loved it and most, even those who screamed when their parents left, would be fine by day 2. You had the odd one or two who were homesick but it was never during the day and on activities, enough went on to distract them.
They also do the 'camps' like the spy one that's been mentioned (which I had more fun being a part of than the kids 
), then an Indiana Jones thing which is brilliant, dance camps and loads of other bits. I was at Boreatton which has one of the best ranges of activities, but the places in Devon are good for watersports, in Scotland they're good for getting messy, Caythorpe is one of the largest and has the best range on the East side of England, etc.
I think holidays like this are good for kids, though saying that, what outdooradventurer says is true, some places are sub-standard and PGL, much like other larger companies, are notorious for under-paying staff.
However, for places like PGL, you know that they are overworked and underpaid yet the instructors are always happy, always having fun and love being around kids, and that, to me, is just as important as having tonnes of qualies hanging out of your ears. The instructors are doing the job because it is fun and they enjoy putting smiles on kids faces, not because they are being paid to do it. I remember PGL being very careful with who they had instructing sessions, pairing more experienced instructors with newer ones and more highly qualified instructors would be paired with less qualified ones. And for things like paddlesports, rifle shooting and quad biking, it would be extremely irresponsible and illegal for the companies to put unqualified members of staff in any form of responsibility on these sessions.
I went to many interviews around the country to the more independant centres and some of their practices are even worse than the large companies. Whilst completely legal and fine to use, one in particular had a very dodgy system on Jacob's Ladder, meaning one instructor holding three ropes and if that instructor let go the kids would fall due to the nature of the knots they used. Yet it was deemed safe by the people who inspect them 
I'd second Mill on the Brue though, I never went there myself but loads of people I know who've worked there or just been have loved it, I was offered a late interview there and if I didn't have my heart set on nannying I would most definately had gone for it! I've only ever heard good things about that place which is very rare in this industry!