I guess the killer question is then:
1) find the demo car you want (by searching
yourself not getting the dlr to do it)
2) call the dlr and try to arrange a test drive (of any car)
If they agree to let you out by yourself, then there is a
risk that the car may have been thrashed.
If they only agree to let you out with a salesperson, then there is still a risk, but it's probably lower
Personally, I'd have no issue with buying an ex demo if the price was right, and it had new tyres etc (or again, that was reflected in the price).
At the end of the day, a demo will be at least looked after by the dlr, as the last thing they want is a demo car that puts prosepective buyers off...