I run win2k/win98 on a a7v133 with fast-writes enabled.
Performance wise, fast writes make about another 15% boost on my frame rate. Just enough to smooth out Unreal Tournament when 15 people are firing rocket launchers. At first, the drivers were really unstable though, and I hear some motherboards just don't like fast-writes at all. I had to install the drivers in the right 'order' to get everything to work.