I have one for my AR and it works quite well. Takes a few hundred rounds to get the hang of it but the accuracy is not much worse than full auto.
For the people saying pay the tax stamp and go FA some numbers.
Full Auto AR: $12,000
[Feel free to correct me if I am too low. If I am too high by all means point me towards the bargain machinegun]
So essentially I will take that slidefire and a few thousands of rounds and practice. And I still come out ahead of the well heeled gentlemen that chose the FA + the paper colonoscopy you have to go through to get it.
And it's nice having a potential autofire option. Nothing says get the hell away from me like a 30 round mag dump.