They certainly whip the cappings off although they can be a bit snappy. There was a thread on one of these on the ɃɃĶĂ form a while back.
I've had no problems with mine and am quite happy with it. its easier than using an uncapping knife. I put a cardboard splash screen on the front of mine as the brush is a above the container the cappings fall into so you can get spattered if you don't wear an apron.
You do have to have a method of dealing with the wax from the cappings though!