I initially bought these to attach to the back of a large phone because I wasn't happy with the card cases I found. When these arrived, the first thing I noticed was that they hold an unbelievable amount of cards. More than any of card case I've found. I loved the colors, and was more than happy with my purchase. My only issue was the adhesion. I just wasn't sure I could trust them to stay attached to the back of my phone case, so I used some ingenuity. A small amount of Gorilla Glue on the back of the card case, and it's stuck until I choose to remove it! Voila. No more issue!
UPDATE: I ordered another set after a change in phones, and I'm even more amazed by these little guys. I stand corrected. They not only hold and unbelievable number of cards but they DO hold tight on their own, with no help from another adhesive. Highly recommend.