For our trip to Kentucky, we used car charger for the iPhone, a cassette adapter to play music, and the MotionX GPS app. It basically worked. Having the GPS integrated with the phone allowed tricks like having the GPS app pause music playing from Pandora in order to announce turns more clearly. But we found that the iPhone would often complain about losing its GPS signal, so we had to leave the iPhone on the dashboard of the car with wires trailing to it.
So for our New England trip, I bought a windshield mount, the Magellan iPhone Car Kit. At first, it seemed like it would be a great advantage, but after a week of use, we were so vexed that we ended up returning it.
The good news:
• It strengthened the GPS reception to the point that we only lost our GPS signal in tunnels.
• The suction cup adhered strongly to the windshield; it didn't move at all over the course of the week.
• Being able to see the screen at a glance made it much easier to understand the directions we were given.
There was lots of bad news, though:
• When we connected the mount to the stereo, there was a staticky buzz whenever the iPhone played sound.
• The kit somehow prevented the iPhone from pausing Pandora in order to announce directions.
• It seemed to take longer to get a fix on our location and orientation. Often we would have to start driving and hope that the GPS figured out where we were and could reroute us successfully.
• The mount seemed unable to charge the iPhone reliably. On several of our trips, the iPhone would slowly discharge over the course of the trip.
• On several trips, the car kit would intermittently glitch out, both failing to charge the phone and silencing its voice directions. The effect was like being guided by a college student who's so sleep-deprived that he's falling asleep three seconds out of every ten.
I still feel that in my perfect world, it would be handy to have my iPhone be my GPS, because it would be integrated with my music and my cell phone and my contacts, and it would be easier to update software for the iPhone. But I do not feel that I've found a good solution yet; perhaps at this time, a standalone GPS unit is a better solution.