I have been playing with it on my build tablet for a few hours now and it's amazing how quickly you get used to it and feel comfortable using it. There are still lots of rough spots, but on a tablet/slate, I think the UI absolutely makes sense. I really do think that the Windows 8 tablet/slate story is going to be a good one, and finally could give the iPad a run for its money (eventually).
In fact, earlier I picked up my iPad and was trying to cycle through the apps by dragging from the left and lost count of the amount of times I tried to open the charms bar! I've already had a couple of ideas for apps I want to make and have been sketching some storyboards for the last couple of hours.
On a desktop though - I just don't know. Admittedly I haven't installed it on a ''proper' pc yet (it keeps crashing out on my home pc during installation, so I've given up for the evening).
I honestly can't see this being a sucess in business at this stage. I'm willing to keep an open mind though, but thinking about my end users at the office, they're going to struggle with the new UI.
I also want to know more about how it is with multiple monitors - ie, can metro apps only run full screen on a single monitor, or can you target one metro app on one screen and another on a second/third screen...? Once I get my home PC set up, I'll be evaluating this more closely though.