I had someone try to sell me once. I declined. It seems like it would be good for having fun at the range but, tactically, I think it has limited usefulness. I’m not saying it has no value at all mind you, just limited usefulness. I have never used one though so I could be wrong. Tactically, it would be of very limited use, since I doubt you could get accurate fire out of it. And I would not want that stock at all if there wasn't a knob or switch to lock it in place for accurate semi-auto fire.
The sound of full-auto fire, or better yet, the sight of a line of bullets stitching across their windshield, WOULD likely give a group of thugs pause, though...
It just seems like a tool designed to waste ammo at the range. I doubt it would work from a prone position which means you would have to stand up to lay down suppression fire. That severely limits it's usefulness. At this point, I think tools designed to squeeze the maximum amount of accuracy out of each round make more sense than tools designed to waste as much ammo as possible. Ammo is gold and an accuracy advantage equals standoff capability which is a fine advantage to have provided you choose your battles wisely.