To be honest its really all about power! If your processor has enough bang then Vista does run well, albeit at a premium over XP. I think many of the retailers have caused this problem by shipping an awful lot of computers that really do not have a lot of performance. This is to my mind why so many people have complained. IT IS a very hungry OS, but in return looks and feels much better than XP. Dont get me wrong, I have XP on one machine and Vista on my main unit. I can use both without feeling hard done by.
But darn it with a 3.5GHZ quad core why would I worry about performance! Vista is perfectly quick on this machine.
As newer machines get more power, this will rarely be a debate I think. Anyway, as long as the machine works well, who cares which OS you use.