Tap the button in time with a song. The light on the button indicates how accurate the measurement is. (Red is bad, green is good.) Once you have an accurate measurement, you can double-click on the digits to set the tempo in iTunes. When you want to measure another song, just start tapping the button again.
TROO Does what it says. Useful if you work with mp3 clips in Garageband or if you want to make smart playlists based on tempo (i.e. workout, etc) Would be nice if it could guess at the tempo and add it to the file much like "Harmonic" with lyrics but I know that may be far off even on expensive programs/hardware.