I bought a small cheap one in a car accessory shop and attached it to the clutch fluid container lid (the tape was provided with).
It's reasonably correct, the distortion I have seem to be constant for all directions, so I can compensate for it in my head. The reasons for it could be one or several:
The difference between the magnetic and geographical north pole.
The manufacturing quality of this little POS.
The metal structure around... As the lid is fairly uniform, I suspect that the rear view mirror stalk would be a culprit.
All in all, it has helped me out a few times when the GPS stayed at home.