I've flown YYZ-HKG on CX in both directions in both the J and F cabins on the 777

F is substantially more comfortable than J in my view -- it is a great seat with tons of room and lots of privacy. The difference is big.

Whether it is worth the extra $$$ is in the eye of the beholder. I find the J cabin a little odd looking and difficult during meal service but on a long-haul flight it is perfectly comfortable.

One thing I hate about the new J is that groups travelling together basically have to stand up to talk to (or worse take pictures of) each other -- I often see this at the start of the flight and given the angle and exposure of the J seats it is quite irritating.