Speaking as a Ruby developer, this is very cool. I could see using this for algorithmically generated music and that would be neat. (Speaking of which, I was meaning to make our noisy git repository events into some kind of algorithmic music — this would be good for that.)*

But speaking as a musician with lots of keyboard instruments — if I were trying to compose a piece of music, I'm not really sure why I would compose musical sequences with Ruby when I can play them with a MIDI keyboard, or record an acoustic instrument and loop it. I see that you can input anything you like with the Ruby DSL, but it's not a great UX compared to a piano keyboard.

I guess to put it another way — I would not use this to replace what I use Logic for. But that's just me.

* see also http://listen.hatnote.com/