Well, I've been playing with it (snigger) for over a month now...
Has been 100% stable although I'm not using it as my main work machine so I'm not on it all day... I've switched it off at the mains a few times to try and catch it out though and it hasn't missed a beat.
However, anybody expecting to load it on their PC and gain a large performance boost will be (unsurprisingly I suppose) disappointed.
I suspect that the vast majority of users (like me) don't need a '3D' desktop or all the thousands of other features that have created the bloated operating systems that most people use. However, if Micro$oft optimised the current system (which I'm sure is totally possible) and gave us more performance they'd put a mahoosive hardware industry out of business over night. It ain't gonna happen...