I've done my own time testing with Xgl off and with it on! Of course this is tuned up pretty nice and I have "Middle Mouse Button" set in Rotate plugin, as the "iniate" shortcut! So I can just drag the cube around or spin it anywhere. With some Firefox windows open, Amarok playing songs without a playlist even, a large png in The Gimp working, four folders, couple of terminals, Open Office Presentation, and K3B burning a CD! You know the average set of Desktops we can run on Linux, but not on Winblows (tested Vista 2months, sucks and s..l..o..w...).
With Xgl off it works fine, but does seem s..l..o..w.. switching desktops, browsers are slower, The Gimp seems Gimpy, and visualization stops in Amarok with music playing. Now top that off with trying to fill out a form in a browser with other windows covering it and having to drag it around to get to the information underneath, typing and posting a blog in another browser and keeping tabs on a ktorrent downloads. Well any computer is going to slow down, but I couldn't believe the difference when I did the same exact things with Xgl loaded!!!!
People don't realize that if your CPU is being used to do the majority of the work (including graphics drawing) w/o Xgl, then with it on, the GPU takes over everything having to do with drawing your desktop, 3-D and 2-D graphics, and leaves your CPU wide open to do the actual computing!
On three separate comparable tests I saved almost half the time used without Xgl on the same computer. With Xgl on, instead of slowing down things almost seem to speed up the more you load! The thing with trying to fill out a form on a web page becomes simple with transparency allowing you to lay the form over the information in another file underneath. Amarok playing a song with a full screen visualization plays like it's the only thing running, and everything seems to come alive! Multi tasking becomes a piece of cake! Xgl and Compiz enable a huge productivity increase that I wasn't really aware of and that can't be described, only demonstrated in tests like this!
So yeah if you use your computer like I do, you don't ever want to go back to sleepwalking in a standard desktop setup snoring your way through work! It's just flat out BORING and wastes way too much of my time! I just plain love it and can't get enough speed! I'm addicted!