I'm guessing the process is going to work like this:
1) Cashier rings up my items and says, "That'll be $3.50"
2) I fumble about unlocking my phone, finding the specific app corresponding to this payment network, wait for it to load. With any luck, the cell network coverage will be so poor that the app can't authenticate my account.
3) Hand the phone to the cashier/hold the phone under a barcode scanner while it attempts to read the barcode presented on the screen.
3a) Tilt the phone this way and that to try and get it to read something other than a reflection from the glossy screen
3b) Rub the screen on my sleeve to clear the fingerprint smudges
4) Apologize to the people behind me for wasting so much time.
I've been through step 3a/3b many times already with my significant other's Starbucks gift cards on her iPhone that use the same technology. Truly annoying when you have to hand your phone into the drive through window while they fumble about getting a clean scan. I'm just waiting for them/us to drop the phone onto the concrete during the hand off one of these times.
I thought I was irritated at the Luddites still writing checks at the counter, this is going to be nearly as bad. Removing a credit card from my wallet and swiping takes seconds and my brief use of Google Wallet via NFC took even less time (unlock phone, tap the terminal). For something that is targeted to replace "expensive to process cash", its looking like it'll be far more inconvenient.