I get so sick of the whiners who say OS X isn't as fast as OS 9 and they won't switch until it is. That observation is usually based solely on the GUI performance and not on the behind-the-scenes performance. Quartz Extreme will go a long way toward solving the GUI gap, but, people, there's more to a computing experience than speed.
I, for one, am thrilled with OS X 10.1.5 running on a 466 MHz iBook. I admit that I'd like the GUI to be faster, but then I think about all the time I'm saving not having to suffer through frequent reboots. I plan to clone my system to my Firewire hard disk, install 10.2 on it, and run some hardware compatibility tests. If all those pass, I won't hesitate to then upgrade the system on my internal disk.
OS X Rules!