This app is a very simple way to test your reaction time. When you tap play, the app changes back and forth from red and yellow screens. It then changes to a green screen with a millisecond timer, and you have to tap as fast as possible to get your best possible reaction time.
You can compete with friends by sharing your fastest times via iMessage, Twitter, Facebook, etc.
There are also In-App Purchases to reset your average time and remove in game ads.