I bought a little USB current meter so I can verify the app.
The current values measured by the app were much more fluctuating as the values shown by the little current meter. But all in all the app was as accurate as I thought, which means, it can be used to evaluate charger/USB cable qualities.
* charging -> 720 mA (the current loaded into the battery if charged)
* discharging -> 390 mA (the current used by the phone)
* measured with the current meter dongle -> 1 090 mA
720 mA + 390 mA ==> 1 110 mA which is pretty close to 1 090 mA.