DH loves his Garmin Edge 705- he uses it on his single and on our tandem.
Loves how he can see the gradient, heart rate, speed etc on the computer against the map of our ride.
He brought his off E-Bay as a factory reconditioned model and it works great- software was all updated and all the bugs repaired- even better than a brand new one from some feedback we heard and heaps cheaper.
It's also waterproof in rain and you don't have to buy a seperate casing to protect it. Also the maps are preloaded or on memory cards, so no need to have an internet plan to access Google maps. Battery life is great as it only goes into map/colour mode when you want it to, so reduces power consumption.

My vote would be to buy the tool that is made to do the job and leave the iPhone for making phone calls on the ride.