The original Rebirth RB-338 was great. Released in 1996, I remember it being one of the first software synthesizers on the PC that seemed serious and sounded cool. An iPhone version has been around for a while but the iPad version was just released for $14.99 and I couldn't resist trying it out.
The multi-touch is what makes the difference of course. While the screen is a tiny bit smaller than would be optimal - it's hard to select drums without accidentally engaging a button or knob - being able to manipulate multiple controls at once without using a mouse is undeniably fun. You can actually feel like you're jamming to a certain extent.
Ultimately though I feel disappointed and am not sure it was worth the $15. We'll see if I go back to it. Without any sort of MIDI support (sending or sync or export) this is just another bleep-toy that won't really integrate with my existing audio tools.
The 303 sounds are also pretty dated sounding to me (or at least not as fat as modern soft-synths), but the drums are still thick and fun. There's also some ugly digital clipping if you start driving it too hard which is less present in modern plugins.
So, much like the original Rebirth, I'm still waiting for an iPad music app that seems serious and sounds cool.