About the only difference is that the player is OS X native. QuickTime performance isn't as good as on OS 9, but since my PowerBook is 333 MHz, it seems to handle it well enough. I like the active window dragging that QuickTime has to offer. The lack of support for the volume buttons is annoying. I like to listen to MP3s and CDs on my PowerBook and don't want to constantly open the System Preferences just to change the volume.
Classic seems to function okay.
It is compatible with almost all of the programs that I have. It's a bit slower than running in OS 9.1 directly, but it's not too bad. I find I am forcing it to quit more than I think I should. It does not work well on either of my two machines. It is tolerable, but barely.