I stick to XP. XP works great for me, rarely ever had any problems with it whatsoever, and I don't feel the need to shell out £200+ to get what I already have, plus a whole load of extra resource-hogging.
My flatmate has Vista. 64 bit. And it doesn't seem to work *too* badly. But he gets freeze-ups (despite having quad-core cpu/some 9000 series gfx/4gb RAM) admittedly rarely, but they happen. He gets blue screens. The amount of blue screens he got when he first installed Vista was insane. Not to mention the fact he purchased 64 bit off the shelf... then when he got home found out it was 32 bit in the box and you actually have to send off for the 64 bit version (which won't install now, comes upw ith some error then bluescreens every time he tries) is just a rip off.
Forgive me, but that RAM Vista wants to use is far better put towards playing my games, not playing my OS