I spent the weekend downloading and upgrading my main XP box with this pre-RC1 test build. Yes, I said upgrading XP! The good news is that is build is very very good. Not perfect, but very good! I am able to use it as my main box.
I have tested VS.NET 2005, SQL Server 2000, SQL Server 2005, MS Office 2003, and even games (ok I am addicted to WoW). All of these programs worked. The only problem I had was that I wasn't granted access to my old Documents folder, but I would guess that has more to do with the fact that I changed the location of the folder with TweakXP.
When the RC comes out, I think it will be close to ready for prime time. I am psyched.