I am still on XP, and it does everything I want and is extremely stable (for me).
Have Vista on the laptop, only because it was pre-installed. That said, I have had no problems with that either.
However, I am sure I will move to Windows 7 on release. I was actually going to move to Vista last year. The early rumours of Windows 7 actually stopped me doing so. As it is, I will now wait.
IMO, one of the big things is people in general don`t like change. I remember dual booting win98 and XP, and would often boot 98 because it was more familiar. Also, for the more advanced users, you invest time learning about the system, specific registry changes, mods, etc. etc. You spend time tweaking it to suit your needs.
Once you install a new OS, you generally have to repeat the process. Most of us on here will enjoy that. Many will not.
I am currently dual booting XP and Win7, and, whilst I still use XP for my work and gaming, I find myself booting Win7 more and more, just to have a play.