My S4 S-View (original) cover definitely interferes with compass readings. As this is an original Samsung case I find this quite incredible how this has been overlooked.
With the case on and open, my compass app would always point in the same direction.
Gradual movement of the front cover from the open to closed position would quickly alter the compass bearing etc.
I decided to ditch the S-View cover for the standard one for the following reasons:
1. Affects compass bearings as described
2. Unable to see notification LED when the cover is closed. (it is in fact not enabled at all when its closed)
3. I'd rather see my full screen caller ID on an incoming call than the small S-View window. I know if you open the case you will then see the caller ID screen, but I'd prefer to see it from the offset.
Only drawback now is that I hate leaving the screen uncovered so I've put a screen protector on, and I can never get to fit one perfectly...I always seem to get one or two specs of dust that cause bubbles.