Microsoft built an OS for hybrid devices. ARM chips can't run legacy apps so Windows RT is DOA. Intel didn't have low-power, high performance chips ready for launch. So there are no good Windows 8 devices out yet.
We'll know a lot more about Windows 8 in a few months as some non-crippled hybrids are released. It's hard to judge Windows 8 on bad hardware. Microsoft should have held off on the release.
No matter how you look at it, Windows 8 should've given a boost to PC sales, even if only for a short while after the launch. But it didn't. I think that implies it's Windows 8's fault, as PC sales were already falling, and Windows 8 did nothing to improve that situation. In fact it may have accelerated the decline.
The execution... Well, I can't comment because I haven't played with a Surface in person yet. Win8 + Metro is a POS though.