Depends on the hardware I think.
I bought a low end laptop with Vista preinstalled, and it runs like an absolute dog.
From boot, it takes about 15 minutes to stabilise and become usable. I tend to get it started, then use my iPhone until it's stopped thrashing.
Basic stuff like right clicking can take up to 10 seconds to draw the menu. Launching task manager takes over a minute. This is without it even hitting swap. Just Google Chrome running, nothing else.
It's waay slower than my lesser specced XP VAIO's, and it's actually slower than the early years Mac I had, the 512. That was in 1986, and it booted and ran from floppies.
My XP desktop machine runs like a Cray in comparison.
Sure Vista/7 is no doubt fine if you've got a decent machine, but if you're running XP on something quite basic, you might regret upgrading.