For me, I format and redo if a) I have the time for it, and b) if I think the system needs it, like if a driver is acting up and it's screwed up a few other things, or if it's just accumulated a lot of registry damage that can't be fixed. I don't like doing that, because while the reinstall of Windows isn't too bad, reinstalling everything else, and restoring all my settings takes a long time.