Hi jrepp,
In reading through this thread, I didn't happen to see any mention of battery powered impact wrenches (my apologies if someone already mentioned them,and I missed it).
We have a Milwaukee 18-volt power plus unit which came with 2 battery packs and a 1-hr recharger. It has a 1/2 inch anvil and puts out 250 ft-lbs torque. This is considerably below the 600 ft-lbs put out by some air tools, but it has been ideal for our automotive and CUT needs. We especially like it for use at the race track. We don't have to haul a compressor, and a generator or find a power hook-up. We go to the track with both batteries fully charged, and they last the full weekend. The recharger can be connected to the truck battery w/ inverter, or you can buy a 12V recharger.
The Milwaukee unit is beefy and well made. We don't abuse it but we use it hard, especially on race weekends. It hasn't failed us yet.
Also, for <font color="red"> SAFETY </font> , I would strongly echo the recommendations of <font color="green"> RayH </font> to use impact sockets, not regular ones. I have had the opportunity to care for two patients (both casual mechanics) who, in separate accidents, were each hit with the schrapnel of an exploding ordinary socket inappropriately used in high power impact wrenches. It wasn't pretty at all. The regular socket material is not hardened, and it is more likely to fracture under use, as opposed to just wearing out.