Yeah, an easy way to say "No coffee, I'm saving money" and then immediately save that money would be a killer thing for me. I try to minimize minor purchases when I'm saving for something, but I like to have my money compartmentalized; you put a few bucks in an envelope (or functional equivilant) and then when the envelope is full, you buy whatever you've been saving for. If there was an easy way to reduce the friction of moving the money into the envelope, I'd probably be way into that.
The goal is to one day have this app moving money when you choose not to spend on the small things. That would for sure make the act of "saving" more real. For now we just wanted to test the concept of tracking when you don't spend money instead of when you do spend.