This past December, day after Christmas, when the girlfriend and I were trying to get out of DFW (the ice storm that wasn't), we had "Valid US Air only" tickets. Numerous flights out of DFW on almost all airlines, including ours, were cancelled. The UA direct to IAD was still operating and we were there plenty early, in fact the first ones on the stand-by list. The UA flight was full and the wait list was over 80 people. When they called our names, the agent questioned our tickets, but accepted them....I think being a 1K (girlfriend is Premier) probably had a lot to do with it.