Impaktor explores some very unique percussion territory, while being relatively user friendly. Instead of triggering this app with on screen pads, or even an external MIDI controller, it listens to your iDevice's mic for impulses. The intensity and transients that result from you banging on shit in your house can make many different sounds from a single preset.
This is because the entire sound engine is driven by synthesis. Rather than samples, the character of the sounds you create near the mic are fed through a synthesis engine; to make new original tones. The engine is quite deep too, from the makers of Sunrizer. If you're not up for diving into the dual-synth engine here, there are a 92 factory presets that cover Electronic, Ethnic, Imaginary and Industrial percussion. I'm deeply impressed with some of their pseudo-acoustic presets.
There is also a great six track recorder, which can have varying loop lengths. You can even overdub new instruments on top to simulate even more tracks. This is very well designed and easy to work with, including helpful options to clear individual tracks or the whole thing. These tracks or live recordings can be exported via iTunes or Audio Pasteboard.
My only disappointment with Impaktor has been the fixed-screen in portrait mode. This is fine on the iPhone, but on the iPad it can be annoying, especially with the Smart Cover and other stands that work in landscape mode.
Disclosure: I received my copy for review from Beep Street.