As others have said, its fun for a range provided you have a good high backstop and no ceiling to worry about (some ranges ive been to don't allow em because inexperienced shooters often put rounds up way high, as with any full auto). but for any real shooting? Lets say this much, if I had a slidefire stock, and the proverbial sht-proverbial-f, well, I'd use the slidefire as a starter log and keep a good old A1 stock. Too much play in em to be truly accurate.
i'd take the 300 bucks and get some ammo and a tax stamp for my new invention... the rubber band... and my stock wouldn't wobble incessantly. if thats even possible. I prefer semi anyway. but if you happen to wipe your windows or your ass with legal tender, than its a pretty cool little novelty item.