On my mid-level notebook running the released builds, I have seen better stability and the UAC now becoming easy to disable, leaving the notification icon (which can be hidden) in the tray. I am sure that if users could easily remove the UAC, then security experts would basically 'slam' Microsoft for leaving a vulnerability.
If I am not mistaken, the UAC is for providing protection for people who don't have any inclination towards learning all the underlying structures of the OS. They just want a reasonably secure OS that is simple to use for their daily work and play. I think Vista comes close to this idea and need.
IMO - Microsoft is timing the release fairly well. It is rather clear that the real performance of Vista needs to be tested using the third party software (programs and drivers) which seem to be difficult to develop without the 'RTM' version.
I would say that it is time for the third party software vendors to show their stuff, so that people can get an idea on what possible issues or great developments are in the future.
I spoke with Adobe concerning CS2, and they made it clear that the Vista platform is not currently supported. Indeed, I would have been surprised had it been supported due to the lack of the 'RTM' version.
It should also be interesting to see if Vista drivers (not beta versions, mind you) become available to users of the interim builds, or whether there will be simple promotion of upcoming drivers for Vista. ATI has released a beta driver that does fairly well, and I have seen marked improvement between build 5728 and 5744 in the same driver package. Wacom has also released Vista beta drivers which have a lot of promise and additional functionality which take advantage of the 'Tablet PC' functionality as well as the new features in Office 2007 for tablet users (both artist tablets and tablet PC's).
Perhaps the question of readiness for release should perhaps be applied to the third party suppliers as well. Without them, the question of whether a new OS ready, needed, or even a prudent choice is too open-ended for a solid answer.
Of course, I am not a coder, so I am sure there are many things which some developers may or may not like about the new OS that would not be obvious to me. 