On a lark I bought this from HF for $14.39 (17.99 less a 20% off coupon), it's currently still $17.99.

http://www.harborfreight.com/air-too...0059-2156.html

I have a couple of cordless impact drivers, one 12V from B&D and the other a 19.2V Craftsman C3; keeping batteries is an expensive proposition. The advantages of an impact driver have been discussed before, notably driving high torque screws by impact rotation which is different than constant torque drill drivers for driving screws and fasteners which require you to provide reaction torque holding the tool. The impact tools use a large number of small impacts balanced by the mass of the tool, it drives much more efficiently and comfortably and avoides breaking the fastener.

Anyway, the air impact driver has a 1/4" hex drive like my cordless impact drivers, and runs off my air compressor so is available for those driveway construction projects without worrying about battery life or even if the batteries are any good. For a whole lot less dinero (That's Texican for money) than the cost of a battery pack and or a cordless driver.

Comes with a male QD air connector and a double-ended slot/phillips bit. I ran mine on a 6 gallon pancake air compressor and a 50-foot 1/4" PU hose.

Seems to work quite well as my cordless impact drivers but I only drove a few screws this past weekend. A bit noisy you have the cool-sounding air tool motor sound as well as the impact sound. Probably in net a bit louder than cordless. Finish is as good as or better than average for HF tools which is say its OK.

Specs say 15 ft-lbs, 8000 RPM (probably impacts per min) and 6CFM. My compressor is less than 6 CFM, but I think it'll be OK since you don't generally use a driver 100% of the time so the tank can store some air.
It has a knob to select direction (L&R, R being CW and L being CCW) and another knob for setting torque which is something even my cordless drivers don't have.

I guess in further testing the torque of this is limited. Spec'ed as 15 ft-lbs which is 180 inch-pounds. MY C3 driver is rated 1500 in-pounds.

The air driver won't put a 2" screw but halfway into an not-pilot-drilled piece of wood. I think my C3 will when working, but right now I have three dead batteries that won't hold a charge long enough for me to test... My B&D impact driver has two battery packs died of old age. Which sort of makes the air tool a winner by default.

But disappointingly low torque for the PSI and air flow.