One other tid bit the WSJ failed to mention about YUP fares:

These are still COACH tickets so if the flight is canceled or a PAX misconnects or whatever, the PAX is holding a coach ticket and may not get a F seat if F is already full on the new flight (other carriers may not even recognize it as YUP). The PAX could end up paying more and still be in the back ... there have been many posts here about that sudden realization. SO, something to keep in mind with the YUPs!

Peace!