I don't see a problem keeping fast zombies, but the L4D zigzag (isn't that where the AI is from btw?) is pretty frustrating. If one sees you, you usually can't hit him until he is right next to you. What i'd like to see is a mix. Some walk relatively slow, some have a jogging speed, and maybe some run about as fast as you. However, the zigzag AI is the only thing I really dislike in the mod as usually the (non existant) mentality of a zombie is to get you as fast as possible, not outmanouver you or dodge bullets.