I haven't used them.... yet. I have yet to think of a way I can use them creatively, as I don't like the idea of replacing my standard sized cards with the mini ones.
I wouldn't want to get rid of the regular size business card but they could be used I guess. Business cards are small to begin with so I am not sure that a smaller version would be kept or not lost.
Wow, they are very colorful - I think they would definitely stand out for some people - however, I would be afraid they would be considered 'campy' and not taken seriously. I feel most people are just wired to expecting the 'regular' size card.
Maybe they can be sent out as a "freebie" in a mailing? Can they be used as bookmarks? That can be a creative way... i've been tempted to get the Moo cards but haven't had the time to actually actively look into them
There are heaps of ways you can use these mini cards which personally I think is a better idea, rather than using them instead of regular sized cards http://uk.moo.com/ideas/minicard-ideas.html
I remember the first time someone gave me one of these, I thought it was the coolest thing ever. I went to a real estate seminar in Atlanta and most of the younger top-producing agents had them.