The first time I used one of these was at a food truck. I was super nervous I would end up paying $40,000 for those gyros or whatever it was I ate that day but as far as I can tell it all worked out OK in the end.
Second time I used one I bought a kiteboard off a fellow who runs a kiteboarding school pretty much anywhere where the wind blows so basically out of his car, I guess.
Oh I think the dude at Reef Wonderland uses something similar too. I remember now I had to use his ipad to pay for an urchin.
Anyhow, from a buyers perspective once you get past the whole "OMG it's just some dude with a phone" thing, it seems like a neat idea to me. I can't speak from experience about the vendor's perspective of things, but it seems to me that they are becoming more and more widely used in any case.