I needed a Windows license for my HTPC and put 8 on it originally, 8.1 now and I've had little trouble with it. I think some of the execution in 8 was poorly done or un-intuitive (especially relating to "Modern" apps being the default program for commonly used files and some of the 'side' menus in desktop), but in principal I like a lot of what they've done with it and where I think it will lead eventually.
I don't think there's anything especially good about the Start menu or desktop UI, it's just something that's been around for so long that it's become accepted and comfortable. It (kind of like QWERTY in a way) was designed just to be something which works - not necessarily something which was designed to work well. Which is good enough for most people - their primary concern is familiarity and comfort (coming at the expense of progress and experimentation). I think there's a lot of room for improvement though and the real potential comes from the fact that Microsoft is such a large company and Windows is so ubiquitous, so if it can get users engaged with new interface ideas and new ways to interact with the PC, it's got a very good chance of reaching into other programs and software as well.
They're foreign at first, but I think ideas like radial menus and mouse gestures could do a lot for the KBM users.