I’ve never thought much about Virtual Desktops in OS X although I have used it frequently on Linux Desktops and at the beginning in OS X Panther.
My thought always was: why more than one Desktop, when I’ve got Expos?. After watching the following video I might have changed my mind.
Virtual Desktops and Expos? seems to be the way to got!

See Spaces and Expos??of the upcoming OS X 10.5 Leopard in action: