A buddy had one that he installed on a S&W M&P-15. I think he spent like $399 on that bump fire stock and I was shocked at the "complexity" of a $400 accessory.
I came away with a handful of opinions.
1) you had to use a particular technique to make it work as designed, and once you figured that out, it absolutely did a high speed mag dump, and it was controllable and not unsafe, wild or out of control
2) it heated up this rifle at an absurd pace, I can't imagine this stock is the nicest way to treat a rifle
3) you would eat piles of ammo quickly, the cost for the average shooter would be a little nutty
4) it was definitely a fun and interesting experience for the shooter and bystanders, and in a fun setting like a side trip from a party or family gathering, it was a fantastic experience for everyone, grins on all faces
For all the negatives, I wouldn't consider having one unless it was one-tenth original price AND it was used on a dedicated "junk gun" where I didn't care about the barrel or, well, any of it. And due to ammo costs, even then it would be used sparingly.
But yes for sure, a very good product with limited use, but fantastic fun.
Beyond that, even the military found years ago that the AR is far better used in semi-auto mode than full-auto mode. Why this overpriced plastic accessory is such a buzzword shouldn't shock me, but like most gun control arguments, arguments against it are ludicrous.