I stumbled on this the other day and found it one of the more useful implementations of the subscription model. My girlfriend's a dental student and I have several dentists in the family, so I know all too well that replacing your brush every 2-3 months is recommended.
I, like most people, end up getting lazy and using the same one for longer than that. A service that saves me the work of doing that and with great prices to boot (brushes start at $2 each delivery) sounds like a win-win to me.
Denarri