This is a clever idea. However, it requires certain conditions to work well--a steady hand, right
lighting conditions, and the participant positioning their hand correctly. I have tried this several
times, and in most cases, it looks rather fake. Because it looks fake, participants immediately ask
if that's an app (though impressed that I still knew their card). If you work in dim lit bars, then
this app will not work well.
Hopefully, they will come out with a new version of the app that
allows you to adjust for all lighting conditions and scale for all possible positions.