I drove down to LA for the weekend with my fancy new iPhone. It makes a pretty good companion on a road trip.
The most amazing change was having a little Internet terminal in my pocket with me. Every driving break I took was a chance to check my email, update my Twitter account, read some RSS feeds, check where I was going on Google Maps. This is a mixed blessing; I love staying connected to the net but the iPhone also isolates you from your local reality. Part of the fun of a road trip is being where you are right there, no matter how odd or dull or uncomfortable or totally cool. I was staring at the Internet so much I was ignoring my surroundings.
It is possible to read email and web pages on the iPhone while driving on a quiet road, but it is not a good idea. Under no circumstances could you reasonably type on the thing while driving.
Connectivity is still a problem. First you have to hook the music output to the car stereo. Thanks to consumer-hostile car manufacturers my only option is an FM transmitter. There aren't any certified iPhone FM transmitters yet; unshielded wiring picks up the GSM tower chatter. But if you're willing to live with a little occasional noise most of the iPod adapters work. The Apple store guy recommended the $80 Monster Cable transmitter. It worked fine, but the construction quality is awful. I'm going to return it and wait for an iPhone approved cradle.
Battery life is a problem. I think the iPhone can survive a full day of occasionally playing music and trying to keep up with far away cell towers, but I was glad to have a way to charge the thing while in the car.
The iPhone Bluetooth is mostly compatible with my Audi A6 builtin handsfree microphone/speaker. Incoming calls correctly paused the playing music and my car picked up the call. There's an annoying noise for two seconds before the call starts. And the Audi has a problem where it only downloads contact info from the SIM card, not the phone memory. Since the iPhone doesn't really store contacts on the SIM I'm left without my phone book in the car UI. Only a minor nuisance.
The iPhone metal case is too slippery and easy to drop. I bought an thick ugly rubber cover so my sweaty hands won't fumble the phone while pulling it out of my pocket. I'm hoping to find a form fitting skin that's only 1mm thick and not ugly.
Overall I really enjoyed having the iPhone with me. It really is a good enough Internet terminal that it replaces a laptop for most functions. I'm still a bit uneasy about being connected all the time; travel should be a time to go offline. I think once the novelty wears off I'll be good about keeping the thing in my pocket unless called for.