VirSyn's iSyn is one of the best software synth and sequencer combos for iOS, and the follow-up, iSyn Poly - Electronic Music Studio, looks even better.
Sadly, it's iPad only, but on the plus-side, the slightly fiddly interface of the iPhone-compatible original has been completely overhauled. It looks much more comfortable on the iPad, fitting onto just three separate screens. The sequencer appears much more sophisticated and now includes quantisation and parameter automation. iSyn Poly will make it possible to record whole pieces of music in real time (using the touch screen or a MIDI Keyboard via the Apple iPad Camera Connection Kit ), and, presumably, the synths are now polyphonic!
For more details on iSyn Poly visit the official page here. We'll be reviewing it in an upcoming issue.