It doesn't seem as if it's been built from the ground up (maybe the core of it is though - I wouldn't know). It has greatly improved from Beta 2 to RC1, which I am using now. There is less "Are you sure you want to allow...?" pop-ups, and stuff like that - but it still does not seem finished. Which makes sense as it is not the final version I'm using.
The windows search is a joke compared to Spotlight, and I think it still looks a bit over the top in many ways. It's not as intuitive as OS X, but it does improve on XP in many ways in my opinion. The file-browsing is quite nice, and there is a nicer "preview" built in. To me the biggest difference seems to be in all the stuff that comes with Vista though: DirectX 10, Office 2007, IE 7.0 a.s.o.
But then again - all of those things, except DX10 will also run on Win XP which is quite stable after 5 years of Service Packs and minor upgrades, and XP doesn't require as many resources as Vista either.
I wouldn't spend the extra money on getting Vista once it gets here though - wait untill Vista SP1.
And not all programs have been updated to run under Vista yet either.
In short, there is still room for improvement, but many of the small things might have been fixed in the final version.
I'll probably be going back to XP at some point, just can't be bothered to format my PC again right now.
Hope it spreads a bit of almost unbiased light on Vista (even though I greatly prefer OS X).
