JAL sets domestic F fare as 10000 yen or 12000 yen above economy fare depending on a route, some deep discounted and group/special fares (such as Japan Explorer fare) do not apply. I think it depends on individual expectations if 10000/12000 yen is worth what is offered by JAL. I think the difference between domestic F service and international F service is true everywhere, it is not just a JAL thing. I have flown JAL domestic F before. JAL domestic F service is very nice. Sure I will not choose F every time I fly JAL domestic routes with F service, but I do not think it is not worth it. I think what JAL offers in domestic F is far better than the U.S. airlines' domestic F service. Over in the U.S., many people easily spend US$ 400-500 above economy fare to sit in an F cabin. I do think JAL offers a better product in domestic F than the U.S. domestic F service, considering the cost of the service.

If OP wants to experience domestic JAL F products during the visit to Japan, looking to splurge a little, then I think it will be a good experience.

P.S.
JAL does not offer F in HND-ISG all year round. F is offered in HND-ISG this August, but no F service in Sept. and Oct. I think HND-OKA is the longest route where JAL offers F all year round.