I got caught by this landing a VC10 at Gutersloh in the late 80s. ATC were giving 400m RVR which was OK for us because our military rules permitted a Cat I ILS at that RVR. We could see the lights from miles away and we made the decision to land when still some distance away (it was my landing). All was well until we reached the flare at which point we hit dense fog and the reflection from the landing lights blinded me. My landing was a bit firmer than normal due to loss of visual references and I found myself doing about 140kt with practically no view out of the front window. The fog was so dense that we missed our turn off but as it was only about 40ft deep, tower could see our tailfin sticking out of the top of it like some sort of landshark and were able to guide us. 10 minutes later the fog was all gone and the whole airfield was covered in a thick layer of dew.
To be frank, I reckon that the quoted RVR was more than a touch "generous" in fact I would be surprised if it was more than 200m and I'm sure that ATC were trying to help us out, but it gave us a false impression of what to expect. That said, if it had been quoted as less than 400m we would almost certainly have made a visual approach because we could see everything we needed from miles away!
The requirements for visual references give a great deal of scope for making that decision to land - the hardest thing could well be getting a civilian ATCO to approve the visual approach on the first place.