I very rarely demo skis before buying. A bit of research from sources (personal, online, publications) that have given me good advice in the past, and I've got enough to trust to go out and buy some boards. Almost never disappointed in what I get (unless it breaks prematurely, which has happened).
I was a bit pickier back in my racing days, but back then "demos" usually consisted of trying pairs from fellow racers. I wasn't of the same physical build as most of my fellow competitors, so had to take their opinions with a healthy grain of salt.
These days, I've been on many dozens of skis from "intermediate rentals" to current race stock to the latest and greatest custom boo-teek layups. My conclusion is that skis are all infinitely better than back in the good old days. I can have real fun on just about anything. The overall versatility of most boards today is actually quite shocking, really. Powder boards that carve and also play well in the moguls - great stuff. I've run beer league races on some true fatties - what a blast!
That said, I think it really depends on the skier, their comfort level, and how much they're able to adapt to what a ski is capable of doing.