As others have already so capably said, there is just that intangible "something" about these rifles that defines comparison to anything else on the market.
I've not had the pleasure of taking one to the firing line as yet, but when you pick one up for the first time, it does make an immediate impression. These don't feel like comparable mid-gas guns, and the weight difference/distribution will leave you wondering how they managed it. They are light -- very light -- and shoulder so naturally that you almost forget that it is an AR-based system. Is the SR series the ultimate evolution of the Stoner design? Well, a lot of folks would seem to think so, including (apparently) the late Gene Stoner himself.
While I realize it isn't terribly instructive to call this one of those "greater than the sum of its parts" situations, quite honestly, I do think that the expression captures it well. Everything about the SR seems to be just a little better than the competition, a little more innovative then the rest, and a little more advanced than standard fare. When taken as a whole, the results are impressive, to say the least.
Stand your ground; don't fire unless fired upon, but if they mean to have a war, let it begin here. -- Captain John Parker, Lexington, 1775.