Windows Vista downgrades needed for some, but not all
I installed XP and Vista on a new PC I built last year (Vista came free w/ the XP MCE 2005 OEM DVD I ordered). I mainly used XP of course as Vista was confusing as hell.
Then after 2-3 weeks I tried to boot into Vista more often, just so I would know about the OS. After a few more weeks I haven't looked back at XP. There's nothing XP can do that Vista can't do better, except of course run on computers with lower specs.
For example someone posted about their frustrations w/ the new Control Panel. Indeed it takes me too long to find what I need (mainly because I never had the patience to read through everything) but that's why there's a search bar nearly everywhere.. if you need to adjust your display just type in "display" and only the display-related control panel options show up.
By the way the Vista install has been substantially more stable than Windows XP, and because I made a computer with decent specifications (cheapest dual core Core2, an NVIDIA 7600GT video card, 2GB RAM 800Mhz DDR2) Vista simply flies. Such a computer can be made for less than $500 USD (including hard drives, power supply, and case).
Admittedly there were many PCs that were being sold as Vista Capable that would have been much better off w/ XP, and Microsoft should be focused on making their OSes more efficient w/ hardware, but it's hardly something to whine about. Rumor is Snow Leopard will only work on Intel macs, and OS X variations significantly raised the bar required for Mac computers (if you wanted it to run well that is).