I'm currently in the process of upgrading Dark, my workstation at work, from Vista RC1 to RC2. It's been at it for about an hour now and it's nowhere near done. Part of it might be that my web browsing and other work related activities continue as it goes, but...
I've heard from several sources that the upgrade process takes forever, but it's hard to really fathom until you've run it. Damn. It's slow. To quote the installer, "Your upgrade may take several hours to complete."
From the little bit I was able to play with RC2 on Wraith last night, I've decided it seems to run smoother than RC1, which is expected. Part of it may be the difference in processors, but we'll see after the upgrade is done. The other difference may be that Dark has a much newer video card and is using the Aero interface, which is prettier but has a higher overhead in performance cost.
Update: Total installation time was approximately 3 hours. Damn.

