I've bought skis off a demo twice.
The first time was a pair of Volkl AC4s (yeah, I know, but I loved'em).
Had them for a day in December 2005 at Alta, then phoned in a buy from Tokyo and they were waiting for me when I got to Jackson Hole in February.
Next are my Stockli AX, demo'd for two days at Davos last February and pulled the trigger when I found a good deal a few months ago.
Hope to get them out on snow in about eight days.
All my other skis have been bought untested, and as the saying goes: they've all been good, just some better than others.
I have to add, though, that in recent years I've mostly chosen based on all the reviews, comments and great info provided by Bears & Pugs. Thanks all