In my experience, only the bigger, destination ski areas in Colorado (Aspen, Telluride, Steamboat, Vail) have as wide a selection of demo skis close to the slopes as the two industry demo days mentioned. Area-sponsored demo days, by contrast, are more limited, as near as I can tell. Some, for instance, offer local brands (Copper Mountain) or a few smaller ski brands (A Basin) only. There is one for women's skis only, I believe (at Loveland?).
On the Front Range, Colo., I really like the two early Nov. industry demo days, one at Loveland (courtesy Christy Sports), one at Keystone (courtesy Colo Ski & Golf); one day each on back to back weekends. This past year, the two were night and day: 5" fresh powder on a good base at the Christy/Loveland demo, skied off rocks at Ski&Golf/Keystone. (Keystone usually gets less snow anyway, especially noticeable early.)
The big ski brand reps have a national circuit they follow, mostly as a herd. They go to demos made available to their biggest selling chain shops, at least in this area. The big brands are all there, with a full line of brand new, perfectly tuned models in multiple lengths - bigger, fresher inventory than any one shop can come close to. And the reps really know their gear, and can advise you straight, on their particular models. Even Kastle was at the last Christy/Loveland demo, because a Christy buyer was thinking of carrying some Kastles this year.
By contrast, in my experience, rentals from a ski area shop around here are great, but with drawbacks: often iffy if any tune on a comparatively limited inventory of what quickly become relatively heavily used demo skis. For me it's often been hard to tell much about the skis given the uneven tune, or lack of tune. Big destination ski shops may be better at this.
(Don't get me wrong, I still love and value a day of demoing skis at my local slope side shop.)
The one big problem with the industry demo days is there are so few of them, so early. (There may be ones I don't know of at the big destination areas, but the two we have Front Range, for me in particular, have been great, especially IF the snow is good.)
By the way, you have to preregister for them - about a week or so in advance, fill out paper work at that time (call Christy's and Ski&Golf to find out how if you're out of state), and get to the ski area involved early, for check in and best selection on a first come, first served basis.