Lots of it. I"m based in Cameron Park where the wx is clear. I shoot
approachs all morning into Mather and Exec when its low. Even though
they are reporting 1/4SM and 000VV I can usually get in about 70% of
the time (with about 15 minutes between attempts). The lesson here is
that if you shoot the approach and see nothing, just try it again. By
the time you get down to 100 AGL you can almost always see the runway.
If you pick up the rabbit at 200 you're pretty much good to go as long
as you have 1/2 vis. Usually you do. Tower's visibility is MUCH
different than your actual visibility.

Fog can be strange. I took off out of Exec last winter with 1/8SM
000VV. I could only see 1 center line strip down. However, after I took
off, I never lost sight of the runway, even at 1000 feet. It was very
easy to see down through the fog, just hard to see through it
horizontally.

-Robert