I've seen it in several catalogs, and I think I read a good review in a magazine, either RIFLE or Rifleshooter, not sure which one it was. It would seem to be a good concept, and I know that some military units are using them now. That tells me that it must work pretty good, or the military probably wouldn't mess with it. This item mounts on the buttstock, not the forearm, so it won't replace a bipod or bag for the front of the gun.
I've thought about getting one, but I only shoot off the bench to zero a gun, then I do most of my practice from field positions, prone, sitting, with crossed sticks, etc. I don't stay in one area too much while hunting, but if you were going to set up and stay in one area most of the time, having to shoot from prone, it should be a definite aid to your shooting. I think that it would be great for sustained varmint shooting from prone, better than sandbags.