I've been using Windows 8 for a few months as my primary work system and I haven't experienced any decline in my efficiency (and, mind you, I'm a very technical user). If you want to live in Desktop mode you can. I never launch Metro unless I want to (typically for quick internet browsing or mail). Beyond that, the only cross-over is in the start menu which, for me, is better than Win7 as searching is faster. You can pin items just as you did in the old menu but with the added benefit of at-a-glance info with Live Tiles.
Another important consideration here is that you're judging Windows 8 in the hardware context of Windows 7 and prior. It can be analogized to running the original Windows GUI on a machine that had no mouse (though, admittedly, not to the same extreme). Windows 8 is wonderful and fluid with touch, and when you use only keyboard and mouse you're not getting the intended experience. Hardware will catch up, I'm sure, but it's not there yet.
Now, I'm not suggesting the upgrade will be an easy transition for everyone. Power users scarcely like change, and I see them being the ones to cling to older versions (just like they did with DOS back in the day). However this is a shift that needed to happen.
Many of you have mentioned this feels like two experiences, that it's great for a tablet but sketchy for a PC. And, as it stands at this exact moment in time, I don't disagree. The bigger picture, though, is that those lines are blurring more everyday. The PC in its current form factor, being immobile and chained to a desk with a keyboard and a mouse, is dying a slow death.
Products like the Surface Pro
represent a very real idea of an ultra mobile tablet which can just as easily serve as your primary machine. Windows 8 is an OS that, unlike every other tablet OS, allows you to be just as productive in a mobile environment as you are sitting at a desk. Have all your files and applications with you at all times, and allow your settings to sync ubiquitously in the cloud.
Like I said, it's a change -- a big change for some (lookin' at you XP users). However I honestly think it's a change which opens up more doors down the road we're going. Otherwise, we'd be left hoping that this whole mobile computing business is just a fad.