I've had Vista RC1 (release candidate 1) for a couple of months. Trouble was, my PC had died so I put it on my son's computer, which had a lot slower processor and not much memory - barely enough system to support Vista. Like most things, "new and improved" isn't necessarily that, and I'm in no rush to buy Vista when it comes out.
Here's a link to a review on CNET that will say a lot more.
In my opinion, If you've got Windows XP, I don't see any real need to upgrade.