Setting it up is pretty straightforward: select the Contactless Mobile option in the app, choose the relevant card and then follow the steps to make the Barclays app your default payment source. Once this has been done, the device only needs to be woken up (so the lock screen shows) and then tapped against a contactless terminal. If a payment is under £30 (the current contactless limit), the payment will be taken instantly, but if it's between £30 - £100, you must enter a PIN before tapping again.
One drawback is that once payments are set up, Barclays' Contactless Mobile service will display a permanent notification on the lock screen that reads: "You can make contactless mobile payments." Barclays says this is to remind you that the payment service is enabled but the only way to remove it is to hide lock screen notifications, which means hiding every notification on your Android device.
If you can put up with that, it should mean you can go out without a wallet or travel without your Oyster card -- something that iOS device owners can already do thanks to Apple Pay.