Loch Rannoch was, I believe, the first-ever timeshare to be built in the United Kingdom. It's situated alongside Loch Rannoch just outside the village of Kinloch Rannoch. I've got two weeks here, one in August in the Lochside apartments - a little bit away from the main complex and a touch closer to the village - and another in September, located on the other side of the hotel, which I use purely for exchange as it's outside the school holiday period (it's also for sale if you're interested). The accommodation is extremely comfortable and some of it has exceptional views over the loch - the view from Lochside 3 across the water is absolutely priceless. The pull-down beds in the living rooms are much more comfortable than the usual sofa-beds so you can confidently sleep six people in a two-bedroom unit, something I would hesitate to do at most places. Both my units have their own saunas too - something my wife values highly, after a day out in the countryside.
Amenities-wise, the resort is a little bare - there's no gym or swimming pool for instance (although that might change in the near future) and very little in organised entertainment. But you can use the bar and restaurant in the on-site hotel or take a short walk into the village, where there's a pub, children's playground, cafeteria (with petrol station) and a well-stocked Spar shop. The last time I was there, in 2007, you could book outdoor activities - from bike and boat hire to some of the wilder sports - through an organisation based in the grounds of the pub. Dogs are no problem here.
Realistically, you'll need a car as it's not easy to get there by public transport, so day trips to Glasgow, Edinburgh, Inverness and Loch Ness are all easy.
Overall, it's a quiet resort, even in the high season - a great place to relax. Butlins it isn't. It's most important to note that the annual maintenance fees are very, very low, compared with most UK timeshares - last year it was around 300 pounds for a two-bedroom unit - although you do have to pay for the electricity you use. It's affiliated to Interval International and Dial an Exchange, both of whom have always managed to organise great exchanges for me.