I'm not particularly musical myself, but lots of people around me are (siblings, friends and coworkers - one group of which are signed to Sony music), so I like to play around with music production software on my iPad (Korg Gadget, Garage Band, Moog Model 15, Odesi, Fugue machine being my main tools + MIDI and Audio routers and USB MIDI controllers). It's a pretty great environment for someone like me (interested, but not very good).

However, a major downside to this setup is that if I close the apps, the setup disappears. That is, my setup uses multiple distinct apps that are connected together (directly, or through the MIDI and Audio routing apps) and there's no way to persist this multi-app setup between sessions, so any time I have to set it up from scratch (because I closed the apps or rebooted the iPad), I lose all at state and have to set it all up again from scratch.

Until that is solved, I don't think I'd really count it as a serious setup. It's an incredibly powerful toy though!

Of course, this applies to an iOS-only (or iOS-centric at least) setup and not what it sounds like you're referring to: using iOS as cintoellets for an otherwise laptop/desktop-based setup. I imagine that has fewer of the above issues.