It's a pretty awful card in terms of earnings and benefits. You'd probably be better served with a Marriott Bonvoy card if you're wanting JL miles.

The US Bonvoy cards only get 2 points per dollar on general spend but have quite a few bonus categories. But if you're eligible for a Japanese version of the card it can be quite good. Marriott points of course do convert to JL and carry the 20% bonus on 60k points transfers for 25k miles.

Having said that, unless you're a resident of Japan or flying JL quite extensively then I can't imagine JMB being the best program for about anything. The award chart isn't good anymore (it used to be), earnings on partners aren't generous, no international upgrade certs for top tier... etc. etc. Frankly even for someone flying JL a lot, I don't see much if any advantage to JL Diamond vs. BA Gold or AA EXP.