It is a waste of ammo IMO, my buddy has a rifle dedicated to the slide fire, it was cool the first three or four trips to the range. Now it stays at home because 100rd beta mag dumps are not that cool.....
I cringe thinking about the cost of the ammo! Maybe if it would work on my .22, but I have heard that there isn't enough recoil on the AR22's for it to work properly.
I was at the range yesterday and a guy was using one. It was cool for a second, then just annoying after the 3rd or 4th mag dump. Plus, that has to hurt his wallet; it hurts mine just watching.