I got one a few months ago and I'm still playing with location. The one I keep going back to is inside the car, driver side, top corner of the windshield, using the suction cup that came with it. Very easy access to it, and it gives a good view of the front end of the car and everything in front of it. Also if angled correctly I can see the majority of the gauges.
But all you have to do is put it in camera mode and take a few pictures where you're considering mounting it, and see which you like best.
IMO I didn't like any other placement of the camera inside the car, unless of course you like watching yourself while you're driving.