The developer's preview has been out for at least half a year. It's relatively stable for something that's supposed to be under development, and even has Windows Updates going for it. I tried it briefly. The interface is weird at first but it's not too bad once you get the hang of it. Learning to use the interface is just as difficult as learning to use any new interface.
It is built like it's for tablets, but there is some functionality that doesn't work so well without a mouse. It's very odd in that respect.
You can use it like a desktop, yes, but the Start menu is gone. Instead, clicking where the Start button normally is takes you to the Metro screen. That's quite a pain. I'm not sure if Microsoft will change that when they start rolling out the OS. The desktop is identical to Windows 7, and is actually just another app.
There are numerous applications built-in that make the whole experience better though, like the RSS reader, which is pretty well-done IMO.
I remember there was a rumor about Windows 8 having virtualization support for all older versions of Windows to allow seamless legacy support. Anyone know if that's still true?