A very close friend of mine had a good experience there. He purchased an e39 M5 from them. It had higher miles than others on the market at the time, but was well taken care of and clean. They worked with him on the numbers so he was buying the car right. He didn't steal it by any means, but when he was done with it he was able to go to Carmax and sell it to them for no fuss and easily put some money in his pocket.
I think a couple of key things that made his transaction favorable....the car was stock, and it came with a BMW CPO warranty (from a previous owner). I'm sure that Lyle and others beat the crap out of that car, but that's what it was made to do, and since it was stock it was made to handle all of that abuse.
So my advice when dealing with Denny would be to not buy a car that has been modified much at all, and yes, most of their crap is beat so look it over closely.