I would make the case that F is a great thing to experience at least once if you can afford it. However I would never consider it worth the extra if the impact on your budget caused you to travel in a lower cabin than Club on another flight.

F offers tangible benefits over J in my experience. These are a better sleep and more personal space. After my limited F flights (3x LHR<>DXB, 2x LHR<>SIN) I never found myself jet lagged. After similar CW flights I did experience jet lag.

I think that both F and J travel is really an investment in your time and peace of mind. If the value you place on these is sufficient to justify the extra cost then go for it.

J buys me complete peace of mind and F provides no more. At best I find that F travel gives me an extra half day of productive time per flight. My standard daily rate is GBP 1k, so provided that the one-way upgrade from CW to F is more than GBP 500, i.e. almost all of the time, it is just not worth the investment for me.