Running CM11 snapshot from 2014-07-08 on my Spring S4, I have serious magnetic compass issues. Either it cannot be read at all (like in GPS Test or GPS Status apps), or very interestingly reads two logical revolutions for every physical revolution (Google Maps). That is, if I point the phone north and spin it around 180 degrees to the south, Google Maps' directional indicator performs a 360 rotation. Continuing to spin the phone another 180 degrees back to north has the directional indicator performing another whole 360 rotation.
The magnetic compass works perfectly in all apps under CM10.2 from 2014-01-31. I've gone back and forth between the ROMs, so I can't imagine it's a hardware issue.