I have a friend who was a 747 pilot at BA and he was lucky enough to position on a Concorde from LHR to JKF to replace a 747 pilot to LHR. Funny enough, when he got to JFK, that flight he was meant to operate got cancelled and he went back to LHR as a passenger again on the Concorde after a night spent in NYC. He loved it but according to him "you couldn't tell the difference".