I've got one. I don't use it much except in the summer, when I wander round the town barefoot quite a lot, and your soles can get a bit hard. I find it quite good because I can never really be bothered to do it manually, and it sort of sands the dry skin off, and then you just put on some cream.
various similar things on amazon which are cheaper. i've got an emjoi one which is pretty good, but if you want a really fine soft finish, after you have used it use a Leighton Denny crystal foot smoother.