Incidently you can switch out the keyboard on the iMS-20 for two Kaoss pads:
The one on the right triggers gated notes in a selectable tuning scale (chromatic, pentatonic, 3rds, 4ths, and loads more) with pitch on the X axis and gate time on the Y axis.
The kaoss pad on the left can use any selectable parameters on either axes.
Just downloaded GarageBand for the iPad (my IQ has dropped several levels) however I must say I was suprised. The idea of the 'playable' drum kits is great fun as it detects how fast/hard you hit the pads/skins and where you hit them. There's three standard drum kits and a couple or so AKAI-like 4x4 pads. Not sure if you can upload your own samples to them yet, would be great if you could, but limiting if you can't. The virtual keyboards are pretty good - looks like they took a number of samples (or limited emulations) from the Minimoog/Voyager, Nord et al in the synth-based tabs. They've been heavily treated to sound radio (idiot) friendly.
Wouldn't have brought Garage Band up but it's like a miniture DAW and the the GUI for each instrument is slick, tactile in use, and well designed.
(If you can stand the cheese).
The iMS-20 is rather fiddly in comparison, at the expense of maintaining a fascia faithful to the original.
Next stop,
Reactable.