Been there done that. A few things to be aware of:
1) iPhone batteries drain pretty quickly running ATT Navigator. Since the C14 has a power outlet, you can just use a car charger to solve that one.
2) ATT Navigator pulls data from the cell network to determine position. Lose coverage, lose GPS. Ask me how I know. I did see a Magellan cradle a while back at the Apple store... something like $100. Might be worth looking into, if it has it's own GPS antenna and map data storage.
3) ATT Navigator is $10 a month. I discontinue the service when I don't need it. It can be added or deleted using the ATT app.
Of course, I have no idea if Verizon has a similar app, so my experience may be of no help. I've used Navigator with my bluetooth helmet in metropolitan areas, and it's quite effective. I wouldn't want to dash mount the phone for visual directions, tho... because the iPhone gets pretty hot when it transmits and receives data that often. Again, ask me how I know. The verbal commands are pretty accurate, and the iPhone automatically mutes music when it gives directions.
So, in a pinch, an iPhone works quite well as a GPS... but nowhere near as effective as an actual GPS unit.