I've got a DeWalt 18V 1/2" impact gun....works great! It'll take tires off of anything I've tried it on, and barely change a battery. I also have to tool set to go with it, so it was an obvious choice. I've also got the small 1/4" hex drive impact, with an adapter to go to 3/8" square drive for sockets. It works great for small bolts and fasteners, as well as running bolts in/out.
Whatever way you go...go with a Li Ion battery setup. They last longer, don't have a memory, can be charged whenever, and are way lighter than any other battery out there.