That's not to say that Microsoft will fail, or that Metro will be worse than the Windows 7 GUI. It's certainly going to be more interesting than the average OS launch, but my gut feeling is that the reaction to this will match the reaction to the Office ribbon.
They didn't replace OS X with iOS, they each exist as expressions of a different type of computing, for different purposes.
Microsoft are attempting to provide one system as a single expression of both types of paradigm, and this is why the jury is out on Windows 8.
It's a bet they may have gotten wrong from a technical debt standpoint.
Now what does Apple do? They've bet on single-user tablets, Intel laptops (with better-than-average but still not great) battery life, separate tablet and desktop ecosystems, and tablets that have only basic productivity apps. How do they bridge this gap and pay this debt?
I think Windows 8 is a very nice platform, and though it has some quirks, I think if people try it they will love it. I'm a Microsoft employee, though, so I'm probably biased.