Had a couple of experiences of them.
I bought a 3yr old Discovery 3 from them. Bit nervous as I didn't even view it, but they were very honest in their description and it was delivered no problems. It was also the cheapest I could find – cheaper even than private sales through autotrader etc.
My Dad also bought a Vectra from them – had a bit of a rattle & turned out to need a complete new exhaust. He took it back within the return deadline (which is 7 days I think) and got his money back. No questions asked, but did take a couple of weeks or so to actually get the money.
Overall my experience is they're pretty good. If you're buying sometihng >3 years old, shop around for aftermarket warranties though – I bought mine from Quake but I think now I could have got it a lot cheaper.