LevelUp has their devices in dozens of merchants in my area (Brookline MA), and as an experiment, I tried using it in as many of them as I could.

So far I have been unable to pay using LevelUp even once. The cashier doesn't know how to use it, or it doesn't have a signal, or it's not turned on, or someone's put a sticker over the camera, or ...

I'd say I've tried 20 or 30 times and had zero successes.