I use mine as both a controller & a input device.
I use midi control apps like MidiTouch, TouchOSC, and Lemur to control My Hardware Synths and Samplers and MSSIAH App to control MSSIAH Mid Synthi Cart on my Commodore 64.
For Music Making on the iPad I use apps like Korg iMS20, Magellan, Animoog, PPG WaveGenerator, NanoStudio, BeatMaker2, Sunvox, Electrify, and Auria to name a few.
For a mobile interface I use a 16bit Behringer U-Control UCA222 that's Powered by the iPad. At home I use A 24bit CakeWalk interface that needs external power.
I use a M-Audio UNO midi interface to sync with my Hardware.