Don't forget your bike is one chuffing huge piece of metal, so I wouldn't want to be using the compass for accurate navigation unless you are able to 'swing' it on a cailbrated base.
I had one that I stuck by the clocks on my 1150 and it just gave a false reading. The only place it gave a true reading, and was still attached to the bike was on the rear rack, which sort of left it usless. So I took it off.
Did a test with a small sucker style compass and it seems that if it is mounted somewhere near the dash bar thingy all is well. But point definately taken - compasses and iron/steel not good.