Forgive me if you already know this.. I was late to iPad. BUT I am currently using Figure (free) and Thumbjam oh and loopy.. and Garageband.
Thumbjam is totally extraordinary and yet simple at the same time: as a means of actually playing (i.e. a variant of a keyboard, or guitar (for those of us who aren't guitarist) its awesome: you can do slides, glisses, tremolos etc..by playing the touch screen like an instrument.
As a simplifier its great too.. as you can actually set the scale you are playing in and the other notes are instantly rendered invisible.. so once your scale is set every note is good.. brilliant for improvising. But also, if you DO plug in a keyboard (using a "camera connection kit" and a mini keyboard ) then you can play the entire range of notes as well.
As a variant of a DAW, or as a sketch pad: you can import (recommended) a drum loop and then sketch out or play other lines as loops of whatever length you choose, potentially adding them into your track as you go. I've been doing a lot of tracks based on looping concepts so haven't been using it for more complex songs tonally or structurally but there would be ways round this..
I still don't know how to get a hardware/software mixer to work with it (please - tips welcome) .. and think that would be really helpful..