We've got a lot of security doors at work, so it's pretty convenient to store my card key in the MoGo2Go instead of in my wallet. Our security cards at work are pretty thick, so I don't think I can fit another card in there but it would probably hold two credit or debit cards easily.
It sticks to the back of an iPhone with some of that fancy 3M stuff, so it won't leave a sticky residue (I previously had a Gelaskin on the back of my phone with the same material, and it peeled off with no problem). Although I've heard that storing cards next to your phone like this can demagnetize them, it seems to only happen to hotel room keys.
If you want one of these for yourself, they're only $4.95 and you can order them online here.