I don't really know if it was illegal to produce Expose for Windows. If it is, then they will be hearing from Cuppertino real soon. If not, then I don't mind. Just take a look at the problems it has in a Windows environment.
Expose has saved my day on OS X. I have a 12" iBook 600 and when I open too many windows I always loose track of them as they tend to easily clutter up my desktop. With Expose, I just press F9 and I get all of my Windows on screen and choose from there. I think that there is a demo on Apple's site. It is practical any way.