I bought a wrist-based heart monitor, all excited to get rid of the chest strap. Had to buy a new cell phone to go with it; my old smart phone couldn't load the app to go with the HRM, it was like "oh, your hardware doesn't support that." True story. That being bluetooth low-energy, it just did the old bluetooth.

Bike GPS units (and GPS watches) use something a little like magic to be easily read in bright sunlight. Your cell phone has pixels that light up to display, and the sun is a lot brighter so it overpowers them. Sports GPS screens are visible because they reflect light from their environment to your eye. (They also draw less battery power that way.)