Windows 8 is a fantastic improvement to Windows 7. Not that I've taken measurements, but it feels much faster. The built-in tools like task manager and even the copying utility are much more informative and useful now. I was much more impressed with it than I thought I would be, and now it's running on two of my family's laptops and our desktop.
Are you saying a desktop user wouldn't want to use windows 8 because of the Metro UI? There's only really one place where I ever see Metro on Windows 8, and that's in the start screen (which replaced the start menu), which is easily navigable using just a keyboard. In fact, I prefer the start screen to the start menu in Windows 7, Vista etc. because it's just a lot faster.
Don't forget the (ugh) Charms bar, and also default apps for any file types for which you haven't changed the associated program yet. Couldn't say whether the start screen is faster than the start menu, for me search results always come up instantaneously, and it's hard to get a lag time below zero. But my computer was reasonably powerful; maybe it's a different story on lower-end hardware.
Anyway, I've upgraded one computer and this was a good reminder to go out and buy a copy so I can upgrade a second computer. I'm personally happy with Windows 8 in that for me the upsides outweigh the downsides. . . but for the sake of fairness I feel obligated to disclaim that I'm both more comfortable with adjusting my habits and more able to adjust the OS's behavior than many people out there.
Long story short, Windows 8 is both a impressive technical step forward and (for desktops) a stunning UI step backward. Which of those matters more is a choice for the individual.
Try Stardock's Start8 utility and you'll see just how useless the Start screen is on a desktop. Start8 is just like a curated Start screen Start menu hybrid. It's a far better implementation than Microsoft's.
Sorry, but I still think Microsoft screwed up by putting a tablet interface on a desktop OS. If they just had spent a little more time on how exactly they planned to get Metro on the desktop, things could have really been great.