I tried the Beta version on my desktop (with XP as the primary system). Within a few weeks, I switched to Win7 as primary. It's startlingly better.
I cheated on the change to RC - I did an upgrade rather than a fresh install; that caused a few minor hiccups but nothing dramatic.
I bought a new hard drive for my laptop so I can switch between XP and Win7 - but the XP drive has stayed in the bag ever since. It's that good.
I'll be installing it as a fresh operating system as soon as the two copies arrive.
It's impressive how it goes off and finds drivers - even for some pretty obscure hardware. It looks like Vista, but it works like XP (only a lot faster).
I've had one nasty moment, when I switched off the "Homegroup" and it locked up the hard drives on both laptop and desktop. Nothing would work! That's a known "bug" - there is a fix, but it involves having another, healthy Win 7 installation to "unlock" the hard drives. I know how to do that now
I keep a USB hard drive caddy for the purpose.