I think there is a lot going for the music ecosystem on ipad - lots of affordable apps, cheap effects compared to desktop etc.
At the same time I don't think the device itself is built with the connectivity for the kind of music making I think about. If all your synths and effects live on the iPad and you don't care about after touch maybe, but if you want to hook external gear up the iPad just isn't made for it.
no headphone out. requires an external audio interface to connect to hardware synths. not an aftertouch capable control surface. Not really portable if I have to lug a bunch of supporting equipment to make it usable.