Well, all I can say is if you're using an older machine with older hardware (doesn't even need to be that old, really, even only a year or two), then XP is probably the route to take.

Myself, I have all fairly new stuff, because I'm in a continuous state of updating. I have an XP 64 bit partition and a Vista Ultimate 64 bit partition, and I can say with 100% certainty that all my apps run smoother and quicker under Vista on this machine. Particularly, my 8800 GTS definitely performs better under Vista than XP, with the same driver release. Especially with Poser. I don't use Poser much at all anymore, but it only takes a minute to see the difference between OGL performance between Vista and XP.
These things are dependent on many factors. You can't just say Vista sux! because it won't run your favorite program well, and you can't say it sux because you haven't learned how to use it.

Vista works, and it works exceedingly well. For most people it works fine right out of the box, for others, it needs some tweaking. Sometimes you have to know what to turn off and what to leave alone.
For those put off by that idea, for those who don't know how to deal with that, for those with mismatched hardware, weak links like cheap RAM, undervolted hardware due to wrong BIOS settings, for those simply not interested in keeping up, then XP is still here, and will be for a while.

For those who are into getting the most out of their system and those who are into buying the latest hardware, and who want the best performance, there's Vista, preferably 64 bit.

XP is an excellent OS. So is Vista. Hundreds of thousands of custom PC builders and those of us who play games and use apps requiring huge amounts of power and RAM can't all be wrong.
One thing I've noticed is the Vista bashers tend to have outdated or mismatched hardware, software, or are often fairly computer illiterate.

Vista is where MS is headed. Windows 7 will probably be alot like it. Stick with XP for as long as you can, but don't go around saying Vista is garbage because you or your system can't handle it and use it.