Currently I am running a dual boot of Windows 7 Ultimate and Windows XP Pro. I run Win 7 for the sheer beauty of the OS. It is a good OS with some great features built in. However, it is still a bit of a resource hog. Not as bad as Vista, but my games still run better under XP due to more available memory.
I run Ubuntu because I started using it on my old laptop after WinXP crashed on my then I tried to dual boot on my netbook but I ended up nuking the HDD so now I run purely Ubuntu, but I want to dual-boot with Win7. *slaps himself* You're so greedy.
Windows 7 Ultimate, it just works. I find it to be a lot faster than both XP and Vista ever were on the same hardware.. so no reason not to be up to date.