These little credit card knives have become popular recently. I was provided one by Muzitao to review, and find it to be a handy little knife that can be easily tucked inside a wallet or another similar place. In the “credit card” formation, the blade is protected, but you can unfold it where the polypropylene becomes the handle and you can cut with it. This is why it is called a credit card knife.
As someone who likes to have knives handy, this is a great little inexpensive blade to have around. One in the wallet, one in the first aid kit, and a few others around. (Besides the one I got from Muzitao, I have a number of others from various places.)
It's not really a “self-defense” knife, but if it is all you have, it would be better than nothing I guess. But it's not made to fight with. However, it will cut. It is a knife, so you won't be able to keep it in your wallet when traveling by air. TSA won't let it through, so you'll need to remember to put it in your checked luggage.
Bottom line: It's a handy little inexpensive tool that can be used to cut things when you don't have anything else. It takes up very little room, and does what it's supposed to.
Here are two more pictures that illustrate how to use this little blade.
The membership pages of this site will have more on these knives, including video.