Some will act on their observed vis. and required visual references, others will say the regulations specify reported RVR. In UK there is the extra barrier of the "Absolute Minima Procedure".
My impression was that the approach ban was introduced specifically to take the decision-making out of the cockpit -- and assessing visibility with any sort of precision from the cockpit is almost impossible. It's unfortunately easy to see what you want to see if you want it enough.