JAL's economy seat is the most best economy seat I have sat in; comfortable, OK entertainment, good storage, USB power, etc. Seat pitch is also quite generous, by modern standards.

Food is also better than on AA in Y.

About the only downsides are: The food (even in Economy) is heavy on prawns so if you can't stand them, book a special meal, but avoid the vegetarian food which is miserable; and the cabin can be rather warm so bring plenty of water and dress lightly.

I would definitely choose JAL 9-across on a 777 over someone else 9-across on a 787.