My most unusual experience in this regard was during the JLN Tornado -- as seen from the Bazaar Cattle Crossing on the Kansas Turnpike, approximately 130 miles away. I could see the storm on the horizon with unusual detail, even cloud-to-ground lightning strikes (although not to the surface). There were two photographers, one with a tripod, set up photographing the usual sight. Unfortunately, as I was by myself didn't feel it was safe to stop.
My guess has always been that the dry air and subsidence behind the storm allowed for the amazing visibility.