There certainly is a difference between F and J on transcontinental flights in terms of the seat (larger, lie flat in First) and the service (more personal in First/slightly better food). When I'm on a paid Biz ticket, I will upgrade, however, I certainly don't think it's worth paying for First on one's own dime.

I will note that there are many threads on the subject. I will let brp do the customary honors of linking to those threads.