The thing that I've used comviz/compositing for is multiple desktops and Firefox; it used to be that redrawing anything in Firefox was about a 2-second operation and by far the most painful part of desktop switching. Compositing means I'm not redrawing, just shifting it in.

Other than that... it's neat, it's nifty, but when you get right down to it it's still pretty much a parlor trick. I'm yet to see a really critical use case that it meets. If Firefox gets to the point where the redraw pain isn't there for me I'd shut it off entirely.