I bought the AS-IV for my M11-9mm years ago. Basically it just put more spring pressure on the trigger so you could bump fire easier. With a fake suppressor on it, it worked pretty well. With a little practice, you don't even need it. Use a big backstop as you're pretty much shooting from the hip and things can get out squirrelly.
The Super-Tac looks similar to something I had for my 10/22 about the same time. It allows you to fire one shot with a pull of the trigger, and one shot when you push forward on the device. You put your finger in the triggerguard and wiggle it back and forth. It did produce pretty good rate of fire before it broke. Not sure how well it work on an SKS with a long or heavy trigger pull.
The GAT and the BMF are trigger cranks and they're probably the most useful and controllable of the bunch. Best used with a bipod or something to steady the gun.
Keep in mind NONE of these allow true full auto fire. They all require you fiddle with something while you're shooting. They'll work to some degree, but don't expect any miracles. I wouldn't pay $30 for them. Do some searching and you might find them cheaper elsewhere.
Kinda funny that their pictures look like scans of 10 year old Shotgun News ads. Jeesh, get a digital camera and post some new pics already!