I knew right away when my OEM RFT went flat after I hit a pothole. Besides TPMS informing me, the car was harder to drive and was also very loud. It did allow me to get home and to the dealer the next day. Of course, they didn't have a replacement in stock, but that's a whole other story. So it was usable, but not very.
By comparison, I reported in this forum when I hit another pothole. This one was hit at a much higher rate of speed, and was with my then new Michelin Pilot Sport A/S Plus tires. Wouldn't you know it, besides some additional vibrations, the car was still plenty drivable at high speeds, but developed a bubble. I shortly got that replaced as well. Long story short, the conventional tire had a better degradation than the run flat. YMMV.
ED on 7/18/2014: 2014 550i, Space Gray, Mocha. M Sport, Cold Weather, DAP, Executive, Lighting
PCD on 9/6/2011: 2011 550i, DSB, black/oyster. Prem 2, Conv, Ventilated, HUD, Sport, DHP, SAT