Personally, I wouldn't use this app because it's clearly the way to centralization and I also wouldn't buy coffee at this point because of the confirmation times, fees, etc.
Clearly, the best solution here would be the lightning network but that's far from being finished.
From what I see, it does the job. And they've moved fast enough to be one step before LN.
Indeed, centralization is bad. But I expect that even LN will be used by a big % of the Bitcoin users in a centralized manner. So at the end, keeping a couple of $ in their app for having at hand for a coffee is okay imho.
I would use such solution if done properly (instant "transaction", which is possible since it's off-chain), and accepted by a big number of merchants I buy from.
(It would be imho a good replacement for the cards that use fiat kept at banks.)