There will always be the arguments for and against the use of mobile phone 'apps' to display a scan-able pass instead of a boarding pass. However, I have come across a new problem.
I was allocated a seat and had a nice electronic boarding pass but the seat was also allocated to another passenger (due a computer glitch) despite being on the aircraft first *I* was the one pulled as the other passenger had a nice shiny printed-at-the-airport cardboard boarding pass and that trumps a mobile phone *EVERY* time. (I am a top grade frequent flier with that airline too)
So if it works - fine, but if there are any problems you are going to lose against an airline issued paper boarding pass.