Well, Compiz is not the real goal. Having fast smooth non-flickering non-tearing graphics and tapping into the power of the GPU is.
The GPU is a powerful friend for doing a lot of things.
On Windows IE and Firefox will render their web pages with the GPU which provides a lot of acceleration.
I'm not 100% sure, but couldn't almost all of things in X be done by the GPU? (Drawing, hinting, font rendering etc.) Wouldn't that be something?
And I grow quite tired of the memory consumption whining. If done right only the memory on the graphics card will be used and not having 512 mb gfx memory laying around dormant is only a good thing in my book.(Win7 already does it that way .. Vista didn't.)
@drag: Thankfully Windows XP support will be with us for years and years to come. Hell, it is still being sold. So before 2015 we don't have to worry that much. Linux is lucky in that regard, but that is just for the fairly broken status quo.