Its recommended you brush your teeth for ~2 minutes. Lets say with this thing you shorten it to 30 seconds. That means you save 3 minutes per day every day on brushing, assuming you brush twice daily. This works out to about 18 hours a year. If your time is more valuable than 17/hr it's arguably worth it.
Time and time again I see people complain about the cost of daily use items but when you factor in how much you actually use them and the potential benefit you can get out of a them it quickly becomes worthwhile.
edit: there is a huge difference between being maximizing productivity and measuring opportunity cost, I'm arguing the latter. I don't expect you to get paid another 18 hours a year. I'm simply pointing out that depending on how YOU value YOUR time spending $300 on a fancy toothbrush is entirely worth it.