Would I pay that many miles for JAL domestic F? Probably not, but if you may never have the chance to fly it again, it's worth a try. The only rival experience I have had on a short flight was QR F class from DXB to DOH, where I effectively drank a full-sized bottle of champagne during a 30-minute flight because they kept filling my glass. JL domestic F is not quite this crazy but it's still a very nice experience. It certainly puts anything in North America to shame.

FWIW, domestic cash upgrades are pretty cheap -- Class J is +2,000 yen and F is +11,000 yen on most routes, and you can upgrade most cash fares (but I believe you can't upgrade partner award tickets or add-on fares). Note that Class J is just a larger seat and doesn't come with any extra service.