I've used them all over the years. I shot my first buck in 1960 using a classic military sling with my arm wrapped around it from standing.
After reading Jeff Cooper's comments about the CW Sling, I had a couple of rifles made to accomondate them. They are much faster than the classic military sling. But they aren't as easy to use for rifle carry as the military design.
Currently I'm using two different Ching Slings. One is the Safari design which uses only two studs. The other is the original design which requires a third stud just ahead of the floorplate. Both work quite well.
Cooper's testing found that from kneeling or sitting, a good, properly adjusted shooting sling will cut group sizes by at least 30%. What I'd add is that when out of breath, a shooting sling will allow me to hit stuff that I'd have very little chance of hitting w/o it. E
