I second that. I have a 600 MHz iBook G3 withOUT Quartz Extreme. When running Panther it feels far snappier than my 800 MHz flat panel iMac G4 running Jag. The iBook boots faster, opens windows faster, opens contextual menus faster, scrolls smoother, shuts down faster, etc, etc. And all this is without the benefit of QE.
Many apps are faster in Panther too. iCal is noticeably quicker, Mail is worlds faster, Sherlock is faster, System Profiler is way faster. Regular apps seem to boot a bit faster too. Sheets come and go faster and with cool effects.
Overall, just the speed increases alone are worth the $129 in my opinion. It's a beautiful OS that is maturing nicely. Unfortunately for Apple, it will probably help to keep me from buying a new Mac for a few more years 🙂
-Drew