On my Mac Pro tower, I run OS X (10.4.11), Win XP, and Vista.
XP isn't so bad compared to OS X. OS X has more eye candy and is somewhat more intuitive. You have to learn more about the OS to use XP, but once you do that, it's snappy and responsive.
Vista, OTOH, is ponderous and slow (8 core quad processor Intel Xeon, 8GB of RAM). It's like driving a truck instead of a sportscar. And it's always inserting itself into what you're doing, by connecting to the internet, or asking you if you really want to do what you are trying to do, etc, etc.
I would think, for experienced XP folk, that Vista would be a step down.
Read This. Guy "upgrades" from Vista to XP and review's the "new" operating system.
Pretty funny for Windows users anyway...