let's not put too fine a point on it, shall we.
if you look through the Polysail website you will see pics and testimonials that clearly show that they can, and do, work and they work well. a single piece of material can be easily cut in such a fashion that it has no choice but to create a foil shape once it is tied to the mast and yards (or boom).
sure, i wouldn't expect polysails to stand up all that well on an America's Cup racing yatch, but i would not hesitate to use them on ANY day-sailer. even trailer-sailers like the Weekender, Voyager, and cabin-built Sharpies would be well served by a suit of polysails.
i used to own a Grampian G-28 that carried $4000 worth of Dacron to make it go. my next build will be a Norwegian Faering that will fly a balanced lug sail made of polytarp that will cost me about fifty bucks and an afternoons labour.
durability? there are testimonials on the polysail site that show that, if properly looked after, some sails are still in service after five years. not bad for 50 bucks and an afternoon in the back yard!
Daren....
ps; i have no affilliation with Polysails. i just think that it's a great idea for small sailboats.