I use a Macintosh for electronic music. While everybody else is moving to OSX, my Mac boots, 90% of the time, into MacOS 9. Don't get me wrong; I'd love to use OSX on my machine (the ability of calling up a UNIX shell in itself is very appealing); however, at the moment it's not yet practical. I use a lot of VST plug-ins (i.e., effects and softsynths), and they're not compatible between the two OSes. In theory, I could make the leap to OSX, but it would involve losing access to my entire library of plug-ins, save for the few which have been ported over. And most plugins are still a MacOS 9-only affair.
Now the disincentive to upgrade is slightly smaller.
Quoted from the most recent Muon Software patch area password mail: