I had never heard of them so I googled them. I found a video showing how to use them.
http://quilting.wonderhowto.com/how-...orners-232843/
I guess if you were really having trouble getting your mitered corners perfect and had some disposable cash, it would be worth the investment. I just think practice makes perfect and I can whip out a pretty darn good mitered corner in no time just from watching videos on the Internet and taking my time.