I may not have been clear. I already have the cadence and HR strap from my Garmin 500.

I'm looking for a way to add dual pedal power data and possibly Nav functions. Supposedly my 500 will give SOME power functions, but is not compatible with dual (and all the other data that goes with that).

I can't justify a new computer right now, especially if buying power meters, so I was thinking the phone would become the computer.