I have Musix Pro and quite like it. There are several options available for custom usage. I don't play the internal sounds very often instead opting for the MIDI out to other synth apps.

While Musix Pro is a nifty controller app, I much prefer Geo Synthesizer:

http://www.wizdommusic.com/products/geo_synthesizer.html

which is a controller that allows for more playing variation and has internal sounds that I find more appealing than Musix Pro.

Good luck and enjoy when you get the time.

See Personal Bio for system setup

My SoundClick Page