After the FOSS coference last week, I installed VirtualBox on my IMac. I then installed both 64studio and Dynebolic (both custom Linux Audio distros). I was pleasantly surprised to see how good the performance was. Sound worked without a problem and MIDI files played in real time without a glitch. I'll need to try doing a digital audio recording and see how that comes out.
If you want to use Rosegarden and Ardour and you have a powerful enough Intel Mac, it looks quite viable inside of VirtualBox on OS X.
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