I just finished reviewing the Korg iPolysix. I was extremely impressed. You get the sounds and vibe of the original for under $30 - that's one heck of a deal IMO. The fact that you actually get two Polysix synths, the kaoss pads, a mixer and a drum machine for that price is just insane.
Anyway, if you have an iPad and don't already own this app, you should definitely check it out.