Ah, the half-workingness that is Linux. I turned on KDE/Kubuntu and composite at work today. It works better than I thought it would (no crashes yet) but man is it buggy. Shadows not matching window sizes, garbage on the screen. Fun stuff.
A few tips:

• If you’re using the nvidia driver, make sure to say Option “RenderAccel” “Enable”, or things will be ass-slow
• Use qtconfig (qt3-qtconfig package on kubuntu) to turn off qt’s built-in menu/popup fading effects, as they seem to just screw up the fade-in/fade-out animations