A couple of good iPhone car mounts

Cradle
I've recently found this to be a very effective windscreen mount for my iPhone. The main thing I like about it is the way it's adjustable so it will grip your phone regardless of what kind of case you use. Because it grips the phone at the sides, it's compatible with the extra tall iPhone 5, and also most other types of smartphone.

Another good iPhone mount I've used is the Griffin Windowseat. The thing I found really useful about this one is the long arm which was perfect for my Mazda 2, in which the bottom of the windscreen was a long way away from the front of the dashboard. The cradle size is adjustable so you can use it with a case on your phone, but possibly not with some of really bulky cases. This was designed prior to the iPhone 5 release and is more suited to the earlier iPhone models up to and including the iPhone 4S. I don't think it would fit an iPhone 5.