You can read some of the reviews for car mounts over at Amazon. The thing I don't like about windshield mounts is that they "advertise" the fact that you have some sort of GPS in the car by leaving a ring on the inside of the windshield. My favorite device, before I got my present vehicle with a factory installed GPS, was a bean bag style of mount made by Garmin. It sat on the dash holding my device while in use but I could easily stow it under a car seat at my destination. Amazon also has some cup holder style mounts. I haven't seen any beanbag style mounts yet for the iPhone unfortunately.
YMMV