I have it installed on my Acer TimelineX and I've been using it on and off at home and university.
Using Windows 8 with keyboard and mouse makes me feel like it's talking down to me. A bit of a 'move your mouse and click the BIIIIIIIIIG buttons!'-sort of feel, if I could describe it. I think of myself as a power user, and the bright colours and big buttons throw me off.
I'll admit, I'm still not used to the full screen apps. I like running Windows with lots of windows open and multitasking between everything open at once, and having only two things open at once in Metro - especially one of them in an extremely minimized size - feels too restrictive.
And then switching to and from Metro to desktop feels incredibly jarring, especially with how different they look and operate. I can't say I'm a fan of it. Desktop mode really doesn't seem to work in something like this.
Basically, trying to use it like I would Windows 7 makes it feel very clunky and restricted. Come time for Windows 8's release, I won't be upgrading my desktop.
But, it's not to say I dislike Windows 8. Again, using it with keyboard and mouse feels like a waste, but with those big and colourful buttons, it would be perfect on a tablet. Windows 8 was built for touchscreens and trying to use it without one doesn't let it show its strong points. Those touch gestures to switch and exit applications, for instance, are awkward to do with a mouse, but would be extremely easy and intuitive to do by touch. And especially as a small, single-window tablet, having a bright and colourful Metro interface with only two applications open side-by-side at a time actually makes sense and feels miles ahead of any iPad or Android tablet.
As for Metro, as much as I complained about how bright and colourful it is, I'm surprised at how elegant it is. Clean lines, solid eye-catching colours, and very smooth animations and transitions. And all it is is text and colour. No fancy logos or icons or flashiness - just bright colours and sharp text. Others have tried with all sorts of nonsense and none are as good or as simple as Metro.
For this, I almost regret not getting that HP Touchsmart laptop way back, because it would have given me a better feel for Windows 8, and I think I would have enjoyed using it more my touch than keyboard and mouse.
Overall, Windows 8 is a tablet operating system and trying to use it on a conventional keyboard and mouse desktop doesn't let it do what it was designed for. For sure, I won't be upgrading Windows 7 on my desktop to Windows 8. But, I'll certainly be looking into a Windows 8 tablet for portable use. I've been thinking of getting a tablet, but I'm not particularly fond of the iPad or any Android tablet. A Windows 8 tablet, on the other hand, would be my perfect portable device.
With Windows 8 as a tablet OS in mind, however, I really hope Microsoft doesn't hype Windows 8 too hard as 'the next Windows'. It's a nice operating system, but it's not Windows in the traditional sense and trying to market it like that will have it fail. Power users will have little use for a touchscreen-based operating system. It will need to be marketed and moved with a good tablet in order to succeed.
tl;dr - bad for desktops, ideals for tablets, won't upgrade from Windows 7, would get a Windows 8 tablet.