I've been using the release candidate on my home computer for 6 months now, and it is hit or miss for me.
Pros:
It's prettier-the login screens, etc. all look really nice. Copying dialog boxes, task manager, shutdown options, control panel, all quite nice.
The metro screen itself is also nice looking, but not really extra functional for me. When I hit the start button in W7, I'd start typing the program I wanted, and hit enter when it came up. W8 still does that, so it's kind of a non-issue.
Performance does seem to be a bit better on W8 than on W7, which was very surprising.
Cons:
The metro apps are pretty much universally bad. The pictures app, video app, facebook app, all meh, and take a LONG time to load. Like when I double click on a picture file, it takes 5-10 seconds to load the "preview" app so I can see the picture. Bah.
Desktop is treated as an "app" which is silly, because as soon as you run any regular application, it will take you to the desktop to run it.
I wouldn't go out and buy it, but I wouldn't downgrade either.