JL has done such a terrible job in deploying domestic F on its domestic routes that I'm surprised they haven't lost all their business to ANA premium class. I'm talking specifically about schedules: F is not consistently available on all the Osaka-Haneda flights like it used to be (maybe only half the flights have F class), so if you want a premium seat at a specific time and JL doesn't have it, you're out of luck. Same thing on the Sapporo and Fukuoka routes. F availability is haphazard. If you want schedule flexibility and a premium product, then ANA is a much better bet, as they have premium class on almost all their flights to major cities.
I suppose the bankruptcy has made it impractical for JAL to add F to more planes, but really they should put 10 or so F seats on the 767s that they fly HND-ITM. (Currently only certain 772s have F.)
R.I.P. The Fourth Amendment.