You could have six external battery packs for your iPhone for the price of an Edge 705 with the maps...

I love love love my Garmins, just got a new 310XT for running in fact, but they're for sure not cheap.

What uforgot said about the iPhone. My DH has one, and if he turns on location services (GPS), the battery doesn't even last 24 hours. If you had the screen on for the whole duration of a several-hour ride... I don't even know if the battery would last that long! I'm not sure how sensitive the iPhone GPS antenna is, either.

I just read about a solar charging sleeve for an iPhone - but the PV panel goes on the back of the sleeve, obviously, so it probably wouldn't work in a bike mount anyway.