I have the 15" HiDef powerbook and it would be unusable without Expose'! I use it almost as much as I click on the mouse button, hot corners allow me to access the stuff on my desktop and to find a window I lost.
I am the kind of guy who never closes anything, I have always got 30+ tabs in Firefox and 15+ finder windows open for all the different projects I am working on. I might add that my poor 1.67ghz powerbook is woefully underpowered for my tasks and I am often finding the rainbow ball of death lingering where my cursor should be...