There are plenty of music synthesizers out there, and plenty of them for the iPad. Reactable may be unique in that it has such a complex, but simple to use, and to understand interface. Even as a guitar player, I can see that this has value in that it can give you an opportunity to graphically understand some aspects of modern music (sequencing, filter sweeps, how to create sounds out of filters and noise sources, etc.).
On top of all of this, it is addictively fun to play with.
One of the more obvious aspects of why this is such a powerful music production tool is the fact that it doesn’t use a keyboard, just your fingers…
Reactable mobile for iPhone, iPad, and iPod touch!
http://www.synthtopia.com/content/2010/10/02/reactable-mobile-for-iphone-ipad-and-ipod-touch
Reactable for iPad review
http://www.knowyourmobile.com/appleiphone/ipad/ipad-apps/ipad-music-apps/687752/reactable_for_ipad_review.html
