I have used and subscribed to Piano Maestro from JoyTunes before Simply Piano, but now I am using this most.
The learning approach is very streamlined and improved over Piano Maestro (which has more content when you subscribe, though).
What is extremely helpful to me is that the app will just back up a couple of seconds in a piece when you make mistakes and re-run the part. This is a lot more efficient than going through a whole piece including the parts where you made no errors. I can not stress enough how good this feature is, you need to try it.
Now, when backing up does not help, it switches to "learning mode", slow, waiting for you to hit the right note and with metronome! Just brilliant! In fact, this is what my piano teacher does: telling me to go even slower and tapping the rhythm along...
The selected songs are great as well and there are more coming on in a steady pace.
Using the app with a digital piano and MIDI over USB, but it also works with acoustic pianos using the microphone.
I try to keep up with all the piano learning apps in the AppStore and I am pretty sure this is now the best one.