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