They are adjustable and will work with a case on the phone. I didn't want a removable holder, so I used a satellite radio dash mounting kit and attached an grip iPod holder to the bracket after discarding the vent clip.
Thanks Drex, I like the short neck on that one compared to the long neck on the one I've been using. Are you having any problems with vibration? I've had to dampen mine with small pieces of foam between the neck and the dashboard.
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.