I find it both cool and useful. The digits on my credit card are already separated into 4 segments, so it's natural for me to both read and type them in 'chunks

I find it both cool and useful. The digits on my credit card are already separated into 4 segments, so it's natural for me to both read and type them in 'chunks' of 4. If I need to compare it to what's on screen, it's much easier to see. And it's harder to lose track of which digit you're at and what's next. Or if you mistyped anything - it's visually clear as well because you're likely to change the "balance" between the individual segments (e.g. push one digit to the next segment).