I would use it in the car and that's pretty much where its value lies.
Ideally yes, the iPhone would have some snappy new dual headphone bluetooth set up with voice dial for listening to music/video and making/receiving calls.
I would use my car's built in Bluetooth for that. Now if only I could sync an address book from the iPhone to the Prius, I'd buy one tomorrow. Maybe a Prius hacker has worked it out ... or I could get one of those Mac mini Prius setups.