Hmm, never seen a self-generated air gun like that before. If you haven't got an air compressor setup already, I'd say it'd be worth trying out. If you already have a compressor and proper air drying equipment then IMO you might as well save yourself half the cost and get a hobby gun elsewhere. If the air generation works as it's described, without creating water along with it, then that's pretty cool and is real handy for a seldom-used tool like you want.
One thing I'm curious of is how the powder cartridges work. If they're refillable, then that's great. If you have to buy more once you're done w/what you get in the kit then that could be a killer and a PITA depending how much they cost.