I used the app for the first time today.
Purchasing experience: Very easy. Entered in some data, quickly looked up my stations, ticket was bought in two minutes.
One thing I was interested about was how the T would make sure a ticket wasn't reused. Once you activate the ticket, you have 90 minutes to use it. In theory, I suppose you could go from point A to point B and back again, if the conductor didn't look very closely, but the e-ticket also does list the originating and destination stations.
One frustrating thing: As I was buying my ticket to go home, a call came in just after I'd hit the confirm purchase button. Even though I declined the call, the purchase process was terminated. I had to go through it all again. I'm hoping they can address that quirk in another build.