Personally I think the new start screen is a huge improvement over the old start menu, the metro apps look gorgeous, and the improvements to the desktop side (speed and usability) are significant. I don't think Win 8 will flop on the desktop.
But I don't think it will succeed on tablets like Microsoft needs it to. It will be a disappointment like WP7. The apps won't materialize like Microsoft needs them to, so customers will be stuck with a gorgeous UI that runs a browser, really basic email client, and MS Office. A handful of quality apps, and a somewhat larger smattering of mediocre-to-crappy apps, just like WP7. WinRT is a really huge breaking change in the APIs, so while you can program apps in VB.NET/C# it's a huge shift to do so. Unless your app is pretty trivial you're looking at rewriting a huge chunk of your code. And if you're gonna have to switch APIs and rewrite then you may as well target the iPad/iPhone, since that's where the market is. Microsoft is claiming that devs should target Metro/WinRT because of the sure-to-be-huge customer base, but that's far from a given. There will be a huge customer base on the desktop sure, but that customer base is already served by the existing application base, there's very little incremental market improvement to be had by going with Metro, unless the Win8 tablets really take off, and unfortunately WinRT makes that a much iffier "if".
If you go scrounging around looking for developer experiences with WinRT you find inspiring stories like this Microsoft MVP who finally figured out *how to read a file*: http://www.sharpgis.net/post/2012/01/12/Reading-and-Writing-text-files-in-Windows-8-Metro.aspx
I think it's telling that 6mo after giving away ~3000 development machines at Build 11, there are something like 100 apps in the app store.
If Metro were simply a newer variant of WPF then I believe Apple would be in for a serious fight for first place in the tablet space. Win 8 is wonderful to use on a tablet. But if the apps don't materialize by the hundreds of thousands in short order, then that just doesn't matter. Microsoft should have learned this lesson with WP7, which is itself much prettier and nicer to use than iOS. That they didn't is somewhat depressing. That they doubled down on their error by making WinRT even more of a breaking change than WP7, and even more aggravating to develop for than Silverlight/WP7 is kind of breathtaking.