Just got back from SIN-HND-KIX-LAX trip on JAL (booked as a single SIN-LAX return segment). Ground crew at SIN had no problem ticketing bags through to LAX, but advised me that we would have to recover the bags for customs at HND and recheck at the transfer desk for the domestic segment. This was not a problem, since I had 12 hours at HND. Unfortunately, the ground staff at HND was completely disorganized and confused. The transfer desk refused to accept the bags, telling me to go upstairs to the main check-in counters. The crew upstairs told me I had to go to the domestic terminal. At the domestic terminal, they ripped off the HND-KIX-LAX tags and insisted that I had to reclaim the bags at KIX. Since my arrival at KIX was 23:00, I asked if I would be able to recheck at KIX for morning flight to LAX. They said counter would be closed until 7:00, but I could leave with left baggage (i had a hotel booked in Osaka, but did not want to lug 3 big bags across town for one night). Upon arriving in KIX, left luggage counter was closed. I barely had time to find locker and run to one of the last trains of the night to Osaka.
When I went to drop the bags at KIX before my LAX flight the next day, it took the ground crew 30+ minutes to verify that my bags were not already checked and loaded because HND crew forgot to delete the bags loaded by SIN to LAX.
To make a long story short, do not expect any miracles from the JAL ground crew, and do not expect the staff at HND to know or care about any promises made by other crews. All in all, it was a very disorganized and stressful end to my trip. I usually travel via ANA, but I wanted to try out JAL to compare. Needless to say, I was not terribly impressed.