I'd agree with others that it is the biggest joke. On larger planes, they have special cans for the priority bags and they're loaded with the others. On other planes, I believe the priority bags will be put in a special section in the hold (unless you're connecting, then they go with connecting bags). They aren't the last ones on the plane (on the big planes, if your bag is last on it goes in a special section and is one of the last off). It isn't free because there's a productivity loss from looking through the bags to find the priority ones to get out first (when you've got a couple hundred bags from a flight this can take a while). The real thing they focus on with baggage is making sure that connecting bags make it on the next flight since lost//delayed bags cost a lot of money.