Moog's first app, Filtatron, is on sale now for just a buck!
This Universal effect app is primarily intended as a super Moogerfooger, for you to run audio in through an interface. If you do not have an interface you can import whole tracks from your iTunes library. There is also an oscillator that you can kind of play with, using an X/Y pad.
The primary effect in this app is a filter, which can be envelope and LFO controlled. There is also an Amp with drive and feedback along with a delay. Presets and included samples give you a good idea of what you can accomplish with it.
It can be fun to import a loop and mangle it in there, or turn on the VCO release and jam with the X/Y pad, though I don't know if I'd recommend buying it for that. I would recommend it emphatically if you have an interface to run instruments through though!
Here is a demo of it with a modular synth. The action starts around 3 minutes in.