Hawaii Marukai JCB holder here. The card's fun and unique (at least in North America), but the one thing that gets me is JCB cashback points are awarded on spending at "lumpy" $100 intervals, so you're always taking some sort of small loss vis-a-vis most other reward cards.
That said, this card is wonderfully fun when you're at a place Japanese tourists frequent. You have access to the JCB Plazas with have free Wi-Fi, drinks, and coupons inside them.