Thank you for being willing to test it! This isn't something I'm making money off of, nor is it something I feel the need to ram down people's throats -- I just wanted to try something people hadn't seen before.
If it works, and people like it, great! I'm glad to have helped a little. Please let me know if you run into any trouble (my Twitter and email are on the site).
Unfortunately, for unrecognised card types it makes it impossible to proceed. I have just tried a maestro card number (beginning 6759) and am unable to enter any of the additional details (expiry, etc).
It may be worth adding a generic fallback for cards that are not recognised to account for unexpected edge cases.
I like it I feel that this is one instance where skeumorphism goes well. I can look at my actual card and visually match what info goes where without having to be told. One thing, I think it would be simpler for a user if the card showed both sides at once, rather than requiring the user to flip it to fill out the other side.
One minor improvement I would make is to prefetch the backgrounds for the specific card types (Visa, MC, Amex) so that the swapping appears seamless. This is a great idea and I will totally keep it in mind for future projects.
The "flip to the other side" to fill out the CVV is great. I might provide a second button/link, appearing once the CVV is completed, that says something to the effect of "valid input, next step"
If it works, and people like it, great! I'm glad to have helped a little. Please let me know if you run into any trouble (my Twitter and email are on the site).