I have used it twice to check in, and to be honest it's quite hopeless. I don't really understand why it can't just generate a passbook BP directly in the app, but you have to type in your phone number to have the link sent by sms. Quite useless if you're keeping your phone in flight mode to avoid expensive roaming charges.
Also I don't like the way the destinations are written. For instance on a normal Passbook BP you'll have the airport codes written in capital letters, with the destination name written on small letters on top. E.g IST -> JFK
In the TK app however you have DEPART -> ARRIVE written instead of the airport codes. Also the app doesn't display your Miles&Smiles status and you can't enter the self-service gates in the lounge using it. In other words it's a useless app.