I've somehow ended upgraded on every CS J flight I've had, but for non-CS J, one of the often overlooked differences is that storage space in F is minimal compared to J. F on the flights I have been on has seemed very tired. You get more room than J obviously and the food and service is better. F on an A380 is one of the better F's in my opinion (and there is no CS in A380 J so the difference is more profound). OTOH I've not been on an F flight that has disembarked through door 1 (F door) for a while, meaning in practice F is disembarking after half of J. Getting 2 A380 air bridges to work seems to be a challenge, let alone 3.