I just got an air file and it is way so cool. It is not for big reshaping jobs, but it is much faster, and less work than using a hand file. Good for minor slotting of holes, de-burring, beveling, and so on. If you have some extra change in your pocket, it may be a big help to you.
It came with four files; round with a tapered end, flat with a tapered end, half round with a flat side again with a tapered end, and a double sided flat file. The tapering allows you to work with larger and smaller holes, large flat areas, or tight corners. Want to make a square hole for a carriage bolt? Drill a hole for the shank then use the flat and triangle files to make your corners and flats.
You just hold on and enjoy the ride and the work being done for you.