For me it depends very much on the alternative to F.

As an example, next month I am flying F LHR-LAX on an A380 and J SFO-LHR on a B747. I have no desire to fly J on an A380 with BA and so the upgrade was worth it for me (particularly for the long day flight and the LHR CCR lounge access). Coming home, I'm happy enough on the upper deck of the B747 that I didn't feel the need to upgrade to First.

The A318 and the B747 upper deck are about the only J class cabins I want to fly on BA - 7 or 8 across configurations on the other long-haul aircraft and scrambling over other passengers (or them over you), no thanks.