Ok, had to chime in. Not sure if a pun is intended there.
Anyway, new user - watched a few vids on Youtube, bought it. Sounds are GREAT! Also love the scales... everything sounds good.
Strings are particularly impressive! Keep it coming!
Onto design. I'm yet another professional designer chiming in... been doing graphic design for about 19 years now. My take:
1) I really like the color differentiation in the "keys" - so much so that I would like to:
a) make them brighter sometimes (maybe by user settings?) (Please oh please let us make user skins to submit)
b) create custom spacings
2) The "corner controls"... uh... well, I'd like to see all four up one edge.. some real estate is lost by using corners. Would LOVE to see buttons (maybe even customizable) for common things - instruments, show/hide key controls, etc.
3) You know how the percussion stuff is laid out in a big grid? Would love same type of thing for all instruments if a user wants - and again, some options - of the playing surface. Hexagons are nice

It's pretty intuitive, but the experimentation gets really cool and I think being able to lay out keys differently once in a while would lead to some interesting results.
4) For that matter, I wouldn't hate piano keys.
Last - this isn't a design thing, but volume control via left/right side of screen is needed!
Ok, that wasn't really last. But this is: Saw you were considering an arpeggiator. YES PLEASE!!
GREAT WORK HERE. An inspiring product in a sea full of use-and-delete alternatives. Thanks.
Andrew