I have gotten reported CPU usage as high as 98.8% for Seti using thinned -doze XP SP2
Have seen OSX as good as 98.2% w/ Leopard w/ no special "thinning".
This while stilll having wireless processes w/ a normal GUI active.
I'd say that 98.x % usage w/o having to boot into command line is pretty darn good!
An OS still needs
some CPU afterall