Beryl
I have been using Beryl for about two months. Beryl adds OpenGL effects to the desktop; similar to Areo Glass on Vista. From what I have seen in Vista, Microsoft keeps the effects to a minimium. Beryl effects are much more aggresive and very noticable. Beryl does not add any productivity value to the computer but it makes it look much nicer.
Beryl runs well on my laptop and other slower CPU machines. Most of the processing is done on the video card instead of the processor so it does not pull resources away from the CPU. There are many demo videos on YouTube showing Beryl. Here is a good one that shows many of the effects.
The only issue I have been having with Beryl is that it does not yet support two monitors. My main workstation has two monitors and anytime I enable Beryl on it, I get unpredictable results. Every time it is unusable. I typically have to log in on a terminal and kill any Beryl related process to make my desktop usable again. Other than that it works great and I recommend you try it out.
Here is a link to the install guide for Beryl on Gentoo.