Still seams that very few computers are faster with Vista than XP. So sure Vista is now OK, if you can't get XP drivers, but for anything but gaming, XP still comes out faster.
In our tests, Windows 7 was a few percent slower than XP SP3, but faster than Vista SP2.
So on high end 64 bit hardware, 64 bit version of Vista is slower than the X86 version of XP. So it still seams very clear, anything less than a multicore 64 bit machine, forget about Vista, if you can.
Also with Vista being released in Dec 2006, but networking performance was not good until SP1, in February 2008. Even then SP1 still didn't fix network performance for VISTA to XP/2000, or over wireless. I know I finally gave up on Vista, 2 months past SP1. I still couldn't stream HD videos from a XP box over 100mB Ethernet link on a core duo laptop, that same video had no problems over a 11 mB wiFi link to that same network using a 800Mhz atom w/linux (of course no issue same laptop running XP, Vista had released drivers, XP didn't).
So vista deserves it's reputation for taking over a year to be usable on networks.