Early adopter here -- it came pre-installed on a notebook.
What I eventually realized is that MS is now supporting 3 separate UIs, all with quirks, and all with separate design philosophies.
The classic, window-based UI has been evolving over 15 years; it's straight-foraward, if cluttered. Start button; apps binned to the task bar; random crap on desktop; text-based menu bars; high contrast, colourful design elements.
Ribbons in Office. Similar to windows, but it replaced the menu bars with ribbons. More customizable than the menu bars, but my old eyes find the muted colours, grays on white, and small icons troublesome, especially in Outlook. Runs exclusively in classic UI.
Metro -- which actually comes in two flavours, touch and keyboard / mouse. The touch interface isn't bad, although I personally find it a pain to sort through open apps. But ... I find it hard to stay in Metro. Open up the calculator app, and you end up with a full screen calculator that looks STUPID on an 18" monitor (similar calculator on a 4" smartphone looks great, mind you). Open up Outlook? Back into classic. Further, the apps themselves feature scrolling vertically and horizontally which is ... disconcerting. If there's a pattern as to the reasoning behind H v. V scrolling, I don't get it. While the tiles themselves are colourful (a reference to the classic UI?), the apps are back to scroll bars that are grey on white (Office?). And the Music app is mostly black / grey / white. Weird choice, that, since it removes a design element that can highlight useful information. And, having a whole bunch of live tiles scrolling information on an 18" monitor is distracting, not illuminating.
But Metro with a keyboard and mouse? I know it can work ... but "put mouse in corner and pray" seems like a poor design choice. Further, I'm unaware of any helpful hints within the OS itself about how to use keyboard shortcuts. Seriously, MS made one of the most counter-intuitive UIs I've ever used with a keyboard and mouse, but did an outlandishly poor job of introducing it. First impressions last -- and if the first impression was "rage", good luck to you.
And, finally, my grousing aside, but if MS had released Win 8 with useful, clever, and outlandishly cool apps, we might not really be having this conversation. Instead, MS has my geographical location (Toronto, ON), but the installed apps gave me news, sports and weather for NYC (seriously, they got the country wrong?). Again, it's small -- but it would've been a nice touch if the apps tried to have a local flare because, frankly, I don't care about NYC. At all. The other apps? Music is interesting, especially since it includes free streaming (something of a big deal in Canada), but the interface blends local libraries with cloud streaming not-quite-seamlessly. The other apps, like mail and calendar, suck.
Win 8 is a deeply weird beast. It's fast. It's stable. And I wouldn't recommend it to anyone, especially if you're wedded to Office. The weird blending of multiple UIs is, plain and simple, goofy.
Looking back at my comments. What I think I would like is a small, tablet-sized second monitor for running Metro, connected to my desktop. I'd have whatever I'm doing on the classic desktop open, but could easily glance over and see Twitter updates, incoming e-mails -- a lot of things I use my iPhone for. Weird thing, that.