Many moons ago held over Belfast in gin clear conditions waiting for ILS Cat I RVR on 25.
Eventually asked for 07 RVR - which was in excess of the 600 metres we required. Visual approach was no problem until about 50 feet when it became obvious that the RVR was indeed not more than about 600 metres.
Provided you are used to CAT 1 conditions and resist the urge to stuff the nose down at the last minute to make the visual segment look correct there is no real problem.
In fact it was good to be able to point out to the pax that I hadn't been telling porkies about fog up at 6000' (where the viz was about 50 miles).