I love these things! They work AMAZINGLY for shedding. I keep mine pretty sharp, though, like the edges, you rub the edges against something hard (I use the top of my Swat container, lol) and it rubs away (and makes a lot of smelly dust)
It lasts a lot less long but it works a lot better.
I did come across one horse it didn't work all that well for, though. But only the one, any other horse I've tried it on it was a godsend.