Metro UI in itself is designed for something like a tablet (limited memory, limited multitasking). But it is _NOT_ the main UI in a Windows 8 system. It is basically a Start Menu that is full screen, that happens to allow you some fast access to apps (such as checking email/news) without having to launch a full browser/email client.
Actually, this isn't really true. Metro UI IS the main UI. The Classic windows desktop is merely an app running under Metro. MS has stated this, and the intent is to move users away from classic windows desktop apps to Metro apps. This is one of the reasons why MS limited access the desktop for apps to only Office on Windows 8 RT devices. This is why they removed any semblance of a start menu from the taskbar. This is why they got rid of "Aeroglass." The latter two being removed later in the beta cycle to push more towards metro.
This is the primary reason there is a lot of pushback on Windows 8. MS wants to move users away from the classic desktop UI to the Metro UI. They have even said as much. They have gone as far as to insist that the Windows 8 UI is NOT Metro, but "Windows." Windows 8 is a "bridge OS" designed to move people to the new UI.
That said, With 3rd party apps like START8, to skip the Metro UI, I find Windows 8 to be pretty good. There are many new helpful features that would rock in Windows 7. Native ISO mounting for example.
Ultimately, MS wants windows to be full screen metro apps, using the cloud as storage for data on a platform that will essentially be the same on Phones, Tablets and Desktops.
IMHO, I think Windows 8 will be considered a "failed" product the way Vista was but for different reasons. Keep in mind, Vista sold very well as it was on most new PC's. Vista introduced many of the features that made Windows 7 as good as it is.
In the end, I believe that Windows 8 would have been an instant hit and success if they didn't insist on this transition away from the "Windows Desktop". If Windows 8 gave users a choice to never see the metro UI and have a Win7 start menu if they use a traditional PC or laptop while defaulting to Metro on Tablet and Hybrid machines, it would be about the best OS one could hope fore.
Instead, they are forcefeeding desktop users Metro when it clearly not great for mouse users.