The automatic formatting seems more "cool" then actually useful. I'd much prefer that it show characters as I type them, like pretty much every other text input field.
As opposed to most other text input fields you encounter in the wild, credit cards (and phone numbers) have a singular canonical format. As long as you don't limit HOW it's entered (or pasted), it makes a lot of sense to coerce the input into the canonical format.
At least for the US, the issuer of a credit card (and thus the format) can frequently be determined from as little as the first digit. For instance, nearly all card numbers starting with '4' are Visa.
Most fields don't reformat and add characters like spaces as I type. I personally find it a bit jarring. I suppose reformatting it one the box loses focus isn't as bad, but I'd still prefer you just keep what I type.
Try moving to a country that splits phone numbers different that what you are used to. I grew up with 3-3-4 but live in a country that splits 2-4-4. Subtly annoying.
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).
