Like others, I'm already using the Vista previews which I receive via my MSDN subscription. I'll be developing new (and updating old) applications to work with Vista, so forewarned is forearmed.
(no, not the things between your elbows and wrists, fool... fore-armed!)
I'm pretty 'meh...' about it to be honest. It wasn't just the prettyness of OS X which relegated my home PC use to 'Wintendo' status. It was the whole approach and design of the OS. Vista's pretty. But it's still pretty much a progression of the Windows archetype -- which I'm not all that fond of (for both technical and visceral reasons).
The only areas of Microsoft's business that I truly enjoy are the development tools and the .NET Framework. Like every platform, there's stuff that drives me insane (VS2005 is quirky on big projects, and .NET has some frustrating gaps), but I get a kick out of using them. I'm a .NET Compact Framework developer (PDA's, Smartphones, etc), and MS really has put some great development tools out there for these devices. It still suffers a little from Windows' wobbliness, but that's par for the course.
The Windows Way on the desktop has simply lost interest for me. Unless Vista pulls some serious rabbits out of some big-ass hats, my time spent using, supporting and fixing Windows will remain on company time only.
[edit: my 'Wintendo' gaming PC still runs Windows 2000. The total number of games I've been unable to run because I don't have XP is.... 0. Of my time spent using XP (every waking second at work), the total number of things I've wished I had in OS X is.... 0. Of my month-or-so spent with the latest Vista CTP, the total number of features I've liked enough to want to use it as my main system at home is... (you guessed it!) 0. Microsoft need something revolutionary, not evolutionary.]