For years I've used my Macs to produce and write music - generally favouring Propellerhead Reason for purely electronic pieces, Garageband where real instruments need to be recorded and often using both in combination.
The final step to a recording after mixing down
is mastering to give it that extra zing and dynamics - for this I use
SonicWORX which is an OS9 application.
Initially this would mean copying files to a thumbdrive, moving to an
OS9 machine, processing then copying back but now I complete the process
with VNC over Ethernet.
My OS9 machine for this task is my 800Mhz G4 iMac which sits in my desk corner keyboardless and mouseless.
I activate sharing between it and my Mac Pro, copy the files to be
processed across, then use Chicken of the VNC to open up a remote
session on the iMac.
The audio files are processed and copied back in the same manner they were sent.
So, there's still a place for OS9 and it's easy to include it in an OSX workflow.