I've tried Windows 8 in VirtualBox with some low-end virtual specs. (1GB of System RAM, 256MB of Video RAM, etc.).

It runs at least twice as fast as Windows XP, which this computer's running on at the moment and is quite clearly more efficient, even with about quarter of this PC's actual specs in Virtualbox.

And not only is it technically fast, but the UI is frankly, though I was originally one of the haters after seeing the media, is awesome. I understand hate from those who haven't actually tried it. I despised the very idea of this 'Metro' UI ruining our desktops. O:

But it's plainly brilliant. It works well. It's customizable enough for anyone who doesn't want to rewrite the entire UI. And it's fast, not just technically, but in appearance. It's easy to access anything. Even admin controls have very easy access.

Granted, a few Metro apps, or more like, a lot of them, or even the majority of them, are too simple and are quite plainly designed for tablets.
But the Mail app gave me hope for the future of Metro. Even on this desktop PC, which I've been using for years, Metro works well, especially in apps like Mail. Mail works brilliantly. It's clean, it works, it's got all the functions needed. It provides quick access to hotmail without wasting time opening the site and logging in.

If more apps are built as well as this, and have enough functions to work properly rather than being over-simplified, like Internet Explorer, then Metro could be successful.

And then alongside all that, there's the Desktop. I don't personally like the way it's only an app. In my opinion, it should be as core to Windows 8 on the desktop as the Start Menu is, or even more important. They should be setup to work with each other more efficiently. At the moment, there's simply a stupid looking thumbnail when you hover of the bottom left corner of the screen. It would be better to re-add the Start button, but have it direct users to.... the Start Menu. Yes, this may annoy modders like Stardock who have released start menu mods, which I'm currently trying out to check out their functionality, but frankly, nobody really needs mods like this. Once you get used to the new Start Menu, you'll likely find you prefer it to the old Start setup.

I haven't tried gaming yet, but from various benchmarks all over the internet, it seems like Windows 8 will work just as well as Windows 7 and in some cases, better. And with DirectX 11.1 now released with a ton of optimization changes, I'm guessing gaming's going to get faster and better from here on in.

And yes, I have tried Windows 7; on a laptop. It's good. I know a lot of the ins and outs. But Windows 8 is still a major improvement with it's UI, if not optimization, though it seems from a lot of benchmarks over the internet that Windows 8 is about twice as fast as Windows 7 in most cases.

I'm definitely buying Windows 8 for my new PC build. It's cheap, it's fast, it's modern, it's brilliant.