I don't see one. Don't bother. If you find yourself needing a new OS, grab Windows 8, but if you already have an OS, there's no big reason to make the jump yet. That might change down the road - MS is putting a lot of effort into synergy with all their various devices, and it's possible that new apps coming out will make the upgrade worthwhile - but at least in the near term, there's no reason to run out and buy a copy.