Microsoft's people, especially Steven Sinofsky, have been saying for quite some time now that the Metro UI is not just a UI styling on top of Windows, but that it IS Windows. They didn't just stick it on top of Win7, they made it part of the OS and added the desktop functionality in on top.
The only thing that supports that notion is the fact that you can snap the desktop along side another app. However, whenever you have to do anything of significant importance you immediately get dumped back to Aero. The only important system function that I have seen in Metro so far is Windows Update. Control Panel, Device Manager, and all the other administrative tools are all still Aero.
I'm glad for this actually because it takes way too many steps to open Windows Update because it is a Metro app...
1) mouse to the bottom right corner
2) mouse straight up to the charms settings button (too far to the left and you have to start over again)
3) click Settings
4) click More PC Settings
5) click Windows Update
To get back to the desktop from the PC Settings screen takes multiple steps as well. Either you hit the top left hot corner and find the desktop by working your way down the list of open apps of you have to bounce back to the Start screen and then to the desktop.
...all of this for a system utility that is pretty unobtrusive under Win 7. This is even bad UI design for a mobile device. On my Droid X with Moto blur to get to the settings menu I just press the left hardware button on the phone and tap settings...
There was a learning curve when I got the phone, but its benefits over my old cell phone were huge so the trade off was worth it. Windows 8 just seems to make it more difficult to do the same stuff I've always done. Any "benefits" seem to be tied to the MS store, and are not there to make my experience any better/easier.
The OS grows on you. I hated it initially and I thought MS's downfall has begum. I've reinstalled it and used it for a day now and I've already adapted to the little changes. Once you memorize the keyb shortcuts its back to the old glory with new features.
IMO It becomes more usable/acceptable as I get used to, but it doesn't at this point offer anything more useful. The keyboard shortcuts do help, but I've always felt that shortcuts being more effective than the mouse would be considered a failing on the UI's part. "Just memorize these 100 arcane keyboard shortcuts, and it's completely simple to use", sounds more like the vi text editor than a solid foundation for a graphical UI.