I've got one mounted to the front of an old Farmall M. Other than the tractor itself not being so nice to drive (grabby clutch, loose steering, etc) it's the easiest and quickest way to put in posts that I know of. The only thing I've ever seen anyone around here use an auger for is gate posts where you want a larger hole filled with concrete so the post won't move with a gate hanging from it. All the normal fence posts are driven in. It's hard to get the posts perfectly vertical, especially if you hit a rock in the ground, but it's usually not bad and a small price to pay for the speed with which you can put posts in. Even in our hard clay it only takes a couple of minutes to put a post in.
Attached is a picture of the beast.