I had to install Windows 8 in a VM to look at a reported IE10 issue with our webapp, and I couldn't believe how annoying the interface was -
First off returning to Metro, I I jammed my mouse into the bottom left corner to get the Metro button, then moved it to the centre of the button to get a good click on it (because that's how I roll), and it just bloody disappeared! It was hard to get used to keeping the mouse in the exact lower left, and how the hell are you supposed to do that with a touchscreen?
Then there was MetroIE, I literally couldn't work out how to get the address bar, I tried doing the usual phone scrolling to the top or bottom, nada! It was only after my co-worker was trying random things that they found it, you have to press RMB on an empty part of the page (as RMB is STILL the the context menu on links), who the hell decided that trash and again how does that even work if you're using touch??
Finally it came time to turn the VM off, where's the shutdown button? Is it in that symbol on the charms menu that looks a bit like a power symbol? Nope. Is it in settings? Nope. Oh, wait, what about power settings? Yes that's it, Settings > Power isn't the power options, it's the shut down menu (Where the actual power settings are I don't know or care).
All in all Windows 8 feels like it's been thrown together by idiots who didn't even bother testing the UI themselves, and needless to say I won't be using it on any computers I own ever.