Just did that tonight at CMI. Crystal clear, can see the appch end (to the 1000' markers) and the departure end, but nothing in the middle due to dense ground fog. I aim for the #'s to touchdown around the 500' markers, planning to go below the VASI so as to touchdown before entering the fog bank. Well, I miss the 500' markers b/c I flared too early (even with the GPWS 10' increment call-outs)...the visual picture totally threw me, speeding toward a 30' wall of nothingness, and I touch down just as we entered the fog. Fwd vis was limited to 2 centerline stripes. It was a trip! As we slowed, we crossed an intersecting runway and our primary taxiway (runs straight into the FBO) without ever knowing it. We found the next taxiway, and taxied into less dense fog almost immediately (the tower could now see us) to which twr says "aw man, you just cost me a buck! I had my money on you guys coming off on your usual taxiway!"
It was a cool landing. We had the airport in sight 20 miles out, in the clear...we could see the departure end, crystal clear, but the middle 6K feet were completely obscured. Very interesting!