Be careful with those, a friend of mine was using them and couldn't remove them. It seems that the tirefly and stem welded themselves together. It can happen when 2 dissimilar types of metal are exposed to the elements. There is a technical term for it, but it's been too long since I was in high school chemistry.
He had to have the valve stems removed and replaced in order to get them off. Not saying they are all like that, but be warned.