Usually apps that require direct hardware support seem to be limited in Classic. Oregon trail will run, but without sound. Episode 1 Racer won't install, but will play if installed in OS9 then booted into X.
I say there's no need for Classic today, but someone would contradict me. Maybe they think Oregon Trail is the greatest thing ever, I don't know. But I do know that as a new switcher you probably won't need OS9 at all. I switch in the beginning of 03 and even then I had a hard time finding Classic apps that weren't already made for OSX (only one was VGS, a PS1 emulator).