The screen is dark 95% of the time. Primary use is bluetooth tunes to the helmet on long trips. Secondary use is GPS, mainly in cities but spoken turn by turn directions allow you to keep your eyes on the road. Looking at the screen comes in handy if you miss a turn. You can pull over, look at the map and get back on track.
Other uses while stopped include still and video camera, internet access (think restaurant reviews, lodging info, e-mail, Facebook), route planning, sharing photos.
The iPad is with me regardless of where I go so this just makes it more convenient.