I'm writing to you from Vista RC1 (build 5600) right now.  I've installed it on 3 different machines.

My first impressions: It's a lot more stable than previous major builds (although not 100% stable by any means), and the performance is improved (but still, not quite on par with XP on the same machine).  Release Candidate 1 is a tremendous improvement on the July CTP, which was the last major public release.  (I spent 3 weeks away with the July CTP of Vista on my laptop.  I used it for digital image editing, e-mail, etc.)

Visually, RC1 is striking.

I still feel like I'm being harassed by requests to elevate permissions for certain actions, such as copying files into the root directory of an attached USB drive.  The result is that I'm rapidly becoming desensitised to the "Continue" button.  The behavior is far improved from Beta 2, but still I think this has a ways to go before your average user isn't "trained" to mindlessly click Continue when prompted by User Account Control.

Frustrations aside, I keep discovering new things (so many new things) that make me say "wow, that's so cool, and I didn't even know that Vista could do that!"  When Vista is finished being polished, it is going to be amazing. 

Jim Allchin explains in his announcement when Vista RC1 will be available through the different programs.  If you don't have access to Vista RC1 yet, WinSuperSite has a screenshot gallery. :)  I'd be interested to hear your thoughts and experiences.

[Update: 20 of the "so many new things" mentioned above are enumerated in ComputerWorld Security's Visual Tour of 20 Reasons Why Windows Vista Will Be Your Next OS.]