Once a release gets to the Release Candidate stage, there's almost no changes to the overall functionality after that point - nothing new gets added, nothing old gets ripped out unless absolutely necessary (this is also called "Feature complete"), it's all fine tuning and reducing the code bloat. Most of the code bloat comes from debug stuff that gets yanked out.
So, really, if you install RC1 and use it for an hour, then install the final build or RTM and use it for an hour, you'll pretty much be SHOCKED at how different it is, seriously.
I give a lot of kudos to the developers, coders and programmers on the Vista teams because I swear it's like an entirely different OS from Beta 2 to RTM. I'd almost swear they had a secret "Turbo Mode" switch in Vista someplace and someone just threw it with the RTM build - it's really that much faster and solid.