What’s impressive is that OS X gets faster with each release - while adding features.
Definitely is. The obvious thing is that they've been moving the UI processing to the GPU more with each generation. By Leopard the CPU won't have much at all to do with the UI. But they have done a lot of tightening and tweaking in the kernel to speed things up too.
As far as adding things, that impresses me too. They advertise this massive Core Animation to make things fly around, warp and morph all over the place -- but they're nice enough to make sure that will all happen on the GPU instead of slowing down your system.
I know people running four+ year-old G4s who say it's faster running Tiger than Jaguar. Show me that with Vista.