I just want to point out that after days of using different non-Aqua themes in OSX, I notice my system feeling a lot snappier. Programs appear to launch faster which I think a few things attribute this:
1. Less memory wasted on caching Aqua style textures or whatever you want to call them.
2. Less time and power wasted on drawing/preparing the GUI widgets.
I've been switching between BBX, QNX, and various Swizcore Studios themes. Currently using the QNX theme. They all seem to be more light weight than Aqua. Other than programs launching faster, the responsiveness of windows is improved as well. I'm using Camino and it feels a little more responsive, for example when I have multiple tabs open and switch between them, there's always a pause before the selected tab is brought up but now with these light weight themes the pause is not as long. There's just little tid bits like that that make me feel happier with OSX!
BTW, my machine is an iBook 800 MHz and of course the ATI Radeon 7500. It just felt sluggish with the Aqua theme. Moreover, I did not like the stripes in certain window areas and menus. I've been bothered by it for a long time and now they're gone! Also, menu transparency isn't as transparent which also helps make the text easier on the eyes.