Every time I go back to OS 9 it hurts a little bit. My mouse keeps rolling down to summon a hidden dock when I want to change apps, and now the app switcher seems downright clunky.
OS X is so much nicer to use, and I have really gotten to liking its asthetic (There was a discussion on macnn about if it qualifies as Bauhaus minimalism, and I agree that it does, but that is a bit off topic.)
So I try to use OS X for all that I can, and I succeed, mostly. But The returns to 9 are about once or twice daily, for printing, or (gasp) serious PDF viewing, which is easier in acrobat than preview. Also, in OS 9 IE, which will not run alongside OS X IE in classic, the acrobat plugin actually works. (Yes, much of my school work consists of "serious PDF viewing" reading articles and the like. And despite Quartz, the OS X PDF viewers are terrible, and acrobat is too slugggish in classic).
So for me, it is a mixed bag. When the real OS X 1.0 comes out, I'm buying it immediately, though.
Zach