Fast zombies are pretty fun, but I think I prefer slow zombies. They add more suspense and horror to the proceedings, fast zombies are just for action. I'd prefer a mutilated, hideous slow zombie shambling up to someone out of the shadows while they're not paying attention over a horde of fast zombies charging at someone out in the open so fast you can barely even see what they look like.
Besides, fast zombies make no sense. If something causes dead bodies to start moving around, and they're all decayed and torn up, as you would expect a zombie to be, why would that allow their muscles to get some sort of superpowered boost and allow them to run so fast? Besides, you'd think that zombies wouldn't even have enough intelligence to decide where they want to go that fast.