Unlike the South Korean launch, Samsung hasn't said how many of those users are active. While the $500 million suggests there's some sustained interest, it's not clear how many people signed up for Samsung Pay, tried it once and promptly forgot about it. The real test of the service's staying power will come if and when Samsung releases more stats -- if the value of purchases keeps up with the number of new users, you'll know that the tech giant is on to something.

There's also the question of how well it's faring next to other payment services. Apple hasn't divulged official figures lately, but local coverage of the Chinese Apple Pay launch this week claims that local iPhone owners linked 38 million cards to the service in the space of a day. The company itself noted that it staggered sign-ups to cope with demand. There's no guarantee that many of those customers will stick around, but it's clear that Samsung will have to expand quickly if it wants to become the mobile payment provider of choice.