It's not "stretchy line," even though it stretches more than Sta-set X. It's easier on the hands, slightly less expensive. Better for tying a bowline or other hitch or knot; at least that's my experience with it, using Sta-set for sheets, Sta-set X for halyards.
I wasn't really fond of X's fuzzy cover and harsher feel on the hands. I've switched to VPC for halyards.
That said, I would bet that Sta-set, or similar polyester double braid, is the most popular line for sheets in the US, maybe the world.