As of now, my Power Mac G4 Digital Audio is the only Mac of mine that dual boots OS 9 and OS X. I have the OS 9 partition hidden from the OS X partition (which runs Tiger) to prevent any problems between them. I use Tiger more for just about everything because the other Macs I use regularly don't like talking to OS 9 over the network and I use my Power Mac as a file server. I will boot into OS 9 on occasion because I do prefer it over OS X. I primarily use OS 9 for gaming (an ATI Rage 128 Pro won't let you run many OS X games) and also for making greeting cards in The Print Shop Deluxe. I also use OS 9 for writing when I can, due to the lack of distractions.
I would like to start using OS 9 more soon, as I absolutely love the way it works and perfect (for me) balance between limitation and functionality.