Like many people here, I've seen surprisingly favourable results using autocalc in sieges. I don't normally even try, but I was just about to shutdown for the night and decided to see how well it worked. There's no way I could get that few casulaties assaulting stone walls without autocalc. Not only that, but I had no siege equipment built and it still won (although I assume that my spy let them in the gate).

Rather distressingly, autocalc also seems to give me better results than fighting manually in normal battles. Just goes to show I'm not used to combat in MTW2 I guess. Or maybe in autocalc billmen do something other than standing around uselessly when in combat with cavalry.