Excellent.
What this app does for English concertina players, or people who are interested in the instrument, is provide a great self-contained practice instrument for use anywhere, and by linking it to other apps gives access to a wealth of sounds beyond that of a straight concertina. Linking it with, for example, Thumbjam gives you access to a wealth of beautiful instrument sounds. I can wear headphones and treat myself to playing a cathedral organ on an English concertina keyboard. Or play it out the headphone socket into my big hifi. Or just sit anywhere and play using the iPad speakers.
It only takes a few minutes to orient the fingers to the keyboard if you are already used to English concertina.
I've been enjoying playing synthesiser sounds through Animoog too. Such fun. There are loads of apps that can be played through the keyboard. Look for Coremidi apps with background playing capability.