Theoretically, the Arofly sounds like a pretty amazing little device. It’s tiny and simply screws onto the rear tire valve to measure speed, cadence and power.
To install the 10-gram unit you simply unscrew your rear Presta valve nut to the top and install a Presta adaptor (it works on Schrader valves without an adaptor) and screw the tiny Arofly sensor/transmitter onto the valve stem.
The unit has a pressure-sensing mechanism that measures tire pressure to determine power and cadence. Also inside the Arofly is a gravity sensor which determine speed from rotational frequency. When the rider pedals, tire pressure varies and based on that cadence and power can be determined.
Arofly claims to measure power to an accuracy of plus/minus two percent. That all sounds great.
What doesn’t sound so great?
The Arofly unit sells for $129 which sounds like a steal. But it don’t expect it to talk to your headunit. Arofly makes their own headunit, the A-Plus head unit (required, and movable from bike to bike), sells for $249. In addition, initial reviews look less than stunning.