magnetometers: do they work all over the globe?
You wouldn't be able to use it as a compass exactly at the magnetic pole, as the magnetic field would be vertical.  

You probably already know about the need to correct for magnetic declination, which is the angle between magnetic north and true north.  It varies irregularly over the earth and over time.  Take a look at a declination map (such as this one), and notice that there is no unique value for declination at the magnetic poles.

So the magnetometer works everywhere since it gives you the correct magnetic field X/Y/Z components; you just can't always use it as compass.