I probably wouldn't rely on one for a big show with many channels and thousands of cues, but if anything iOS is more bullet proof than OS X in terms of reliability, and audio output is at least on par with my MacBook Pro. Plus I get super fast, super reliable flash storage, so I don't have to worry about HD issues. Flexibility? I need to run the output to an external device via a mini jack, just like with my laptop, and I get a device that's easy to shoehorn into tight spaces. For all intents and purposes, when running a particular piece of software the iPad become a dedicated device with a huge control surface, with OS overhead kept to a minimum. So while a certainly wouldn't try to edit and process my sound files on one (yet) it seems kind of ideal as a playback device.
I realize the iPad might seem like a bit of a toy, in certain quarters, but for sound purposes it has more than enough processing power and storage space, coupled with an extremely robust OS. This is speaking for myself, of course, not trying to talk anyone into "drinking the Koolaid." Now all I need is the right application to take advantage of the potential.