Air impact wrenches are by far the most popular of all air tools, and if you're doing any serious amount of automotive work you'll definitely want one. Before you rush out and make a purchase however, there are a couple of considerations - power source, and socket size. Power source will depend on your budget, and what tools you already have. Socket size will depend on the type of work you're doing.

Your other main choice will be socket size. Most impact wrenches are available in sizes from 1/4" to 2" and even larger. For the home workshop a 1/2" is usually sufficient, with a 1" drive being more suitable if you're working on SUVs or light commercial vehicles. If you're working on big rigs then a 2" drive might be needed. Larger than 2" is normally only used for industrial purposes.

There is one other choice to me made, and that is style. The smallest wrenches are shaped like a screwdriver, but they are only capable of delivering low torque. Most 1/2" wrenches are of the "pistol grip" variety, shaped like an electric hand drill. As you move up to heavier duty tools you'll start to come across D and T grips which allow for a much firmer hold, and so much higher torque levels. The correct style for you will depend on the type of work you'll be doing, so take that into consideration when making a purchase.

As already mentioned, air impact wrenches are by far the most popular, but that doesn't mean they are your only choice. While they are relatively inexpensive, and offer good power, they do need a compressor as a power source. Unless you already have a suitable compressor that will add a considerable amount to the cost of the tool. If you don't have a suitable compressor, and are doing only occasional or light automotive work (such as in a home workshop) then an electric or cordless wrench is probably a better bet. These days the cordless wrenches are more than capable of light work, and are reasonably priced.