I'm using it as my primary now. Don't go too crazy with installing drivers though, many things not on Windows Update may not play nice with Vista.
Looks nice, IE is now a modern browser, and seems to handle hangups better than XP, but it's still a beta so don't expect full software compatibility or performance. (but hey, it's got a fancy GUI, stability and security, and it runs more windows apps and with better performance than OSX or Linux could ever hope to so I'd say it makes a valid alternate OS right now)