The site looks nice and I get the idea, I think the only problem is that I would see this as way to formal with my friends and family. If I pick up dinner with friends, or if I pay his part of the cab fare.. he just buys me dinner or a beer later. This seems too cold and formal for most uses.
Now for something like parents lending to their kids it would make more sense, or formal loans between friends (which is usually a bad idea)