Perhaps one of the biggest online sites in direct support of all things Windows, and a long-time champion of Windows Longhorn (now Vista),
Paul Thurrott's SuperSite for Windows has come out with a scathing article on the inadequacies of Windows Vista as it currently stands in beta. While Paul picks apart others who have complained about the beta thus far, mostly for being less than intelligent in his eyes, he does agree with their end-assessment, if not with the reasoning they used to reach it.
Many of the points he complains about are how Windows has regressed in terms of application compatibility, from Adobe issues to ActiveX controls on web pages. Probably the best though are the cut and paste COM errors he gets with the Office 2007 beta (since the two are supposedly "Better Together" according to Microsoft). Then there's any number of usability complaints over how it seems to defy common logic, or previous versions of Windows.
In short, not only is his answer to "Is Vista ready?" a solid "No", it's a resounding "No. God, no"
Be sure to check out the rant, in all of it's glory, in the link below.