The Diamond Premier lounge at HND is nothing special, but the inflight experience in JL domestic F is wonderful. Probably the best domestic F service in the world except for certain transcon US flights. If you like premium alcohol, try the shochu which is the Mori Izo brand that retails for JPY 20,000 (USD 140) per bottle.
Personally I enjoy JL domestic F and am fine to spend 23,000 miles per segment for it (the rate on most HND-ITM flights), but only you can decide if it is worth it for you.