Ahem...while "older", I have used similar apps and I can tell you the hardest part about them is actually getting someone to go into them and pay you. Technology doesn't fix that issue. Or the fact that your friend's account is empty and they CAN'T pay you back immediately.
I've had enough trouble with paying up front for folks who took forever to pay me back, even when they had the means to do so. Given my past life experiences (remember, I'm "older" LOL) I'd rather see that everyone actually has the cash up front, before paying for anything.