Win 8 is entirely usable on a desktop once you get rid of any Metro traces. Sadly, that entirely defeats the purpose of what MS is trying to bring to the table, and I'm afraid they are simply too little too late on the tablet front.
The whole Windows on Arm (now called Windows RT) is going to be a debacle.
The kicker for guys like me is: I use Win 7, day in, day out. IMHO, best Windows version yet. If I have to gut Windows 8 and otherwise disable everything "new and innovative" in it to make it a productive desktop environment, why in Christ would I even buy it? I won't.
That being said, this time next year PC's will be coming with W8 on them from OEM's. That's always been the MS gag - they did it with Vista as well. Release a new OS, new PC's come with that, and call it "progress". My personal feelings are that Windows 8, unless radically changed is the next "Vista" in the making. I state this as a MCSE/MCSA. I dread the idea of having to support Windows 8.
I mean that in a public perspective aspect - Vista is widely seen as a complete turd, yet they still did move 10 million copies of Vista into the market every single month for 18 consecutive months, making money on each and every copy, whether it be OEM, VLK, or retail SKU's of Vista..
Does that make said product "good"? Hell no, but the way the show gets played out, the way the OS and OEM system vendor relationships work, they will move a lot of copies of W8.