I use it with my students when I want to get them to learn concepts perfectly. I give each students three or four counters and then I have them say sentences one by one. Each time they say it right, they have to push one of the teeth. If they make a mistake though, they have to push two teeth. If they get bit, they have to hand over one of their counters. It REALLY makes them be careful and pay attention. Even the adults get super apprehensive about it. It's fun to see and it really does a good job getting them to iron out their mistakes. I own three of them. If one breaks, I'll be replacing it.
P.S. If you're not using it right, it is the most boring toy on the face of the planet. If you're using it right, there's nothing more effective.