I've used the Craftsman C3 Cordless Impact for two seasons of racing and love it.
http://www.craftsman.com/shc/s/p_10...word=C3+impact&prdNo=2&blockNo=2&blockType=L2
Use it primarily to bust the wheels off and back onto the Heilig-Meyers Ford. I torque the wheels to 90# and most of the time if the battery is half charge or better, the torque wrench doesn't move before it clicks. On sale now for $89.99+tx & maybe shipping (no battery). $152++in a kit with two Ni-Cad batteries and bag. Unsure it the kit includes a chargers, be careful. I see a few poor reviews on the Craftsman web site, but I don't have any of those problems. The only time I can't get a lug nut off the race car is because the battery is down. Put in a fresh battery and the lug nut zips right off.
I bought a HUGE box of Craftsman C3 Cordless tools when I started racing in 2006 and love them. The Ni-Cad batteries seem to last a reasonable amount of time, they recharge quickly, and the tools work great. Since then I've added a second light, leaf blower (jury out on that right now), drywall rotor zip cutting tool (worked great the one time I needed it and it stands ready for the next time), the impact wrench, and a hedge trimmer. The hedge trimmer is the best powered (corded or cordless) one I've ever owned. As the batterires drain down, I completely drain them, I put in a fresh one and charge (I think I've got four now) the down ones. I've never gotten ahead of the batteries, they are always waiting for me. None of the Ni-Cads have crapped out on me yet; I keep waiting so I can justify buying Lithium Ion.
I see this impact wrench being used more and more at the race track. Highly recommend.