MIDIRack is a MIDI workbench for iPad and iPhone with 6 touch-keyboard types that can sit beside sequencer lanes and your own controls. The interesting part is the relationship between that playable surface and the graph underneath: a knob can control a specific part of the musical process you build.
Start small. The manual walks through opening Harmony Keys, inserting an arpeggiator into its signal path, and binding a surface knob to the arpeggiator's rate. Turning that knob then changes the pattern while you hold a chord. You can hear patches through the built-in instruments or route MIDI to another synth. It also runs inside AUM through AUv3. The iPad screens shown here give the graph and controls room to sit apart.
I like the idea of designing the instrument's front panel as well as its behavior. A September US App Store review, however, described problems with module names and particular processing nodes. In October, a Loopy Pro Forum user reported editor freezes, then said an iOS update stopped the freezes but left occasional slowdown in crowded patches. I'd test the specific routing a project needs before building an elaborate performance around it.
I'd reach for it when a synth has the right sound but I want different controls under my fingers.
