The apps audiobus and midi bridge turn my ipad in a fullfledged soundsystem, with audio routing and midi routing... Using yamaha music sequencer, korg gadget and many other great sounding instruments as well as creative tools...
Music reader, there is many great apps usable on stage as well as at home..
Most important however are lemur and touchOSC apps, that turn the ipad in a fullfledged controller for kronos, tyros5 but mostly for Macbook running mainstage...
My ipad is the most versatile thing i ever owned, next to being versatile its also rockstable and unbreakable