I have them on both bikes - fogs on the K bike and Aux lights on the GS. The Aux lights have a distinct horizontal cut off whereas the fogs are more diffuse. On the GS I have HID conversions for the Aux lights and the K bike they're halogen.
They make a huge difference in my view to visibility. I vary whether they are on or not though - they're never on at night unless there actually is poor visibility and during the day I tend to have them on when there's a low sun behind me. If I remember I also turn them off in town as they can glare when close up behind a car due to the bike's lights being that much higher.
I also have some Bikeviz bullet LED's but don't think they're that effective to be honest - just not enough power.
