Lot of quality remarks here.

It's already been stated that running roadwork is same or better/worse - depending on who's talking.

It's just not true. The points made about resistance in the roadwork b/c the ground does not run under the foot like a mill have already been established and understood. However, I have personally seen guys who do nothing but roadwork, nothing else and their dogs typically cannot breathe like a dog conditioned on a mill.

I put a lot of study to this and my own philosophy is that a mill will condition "air" or "wind" in a dog superior to roadwork due to fact you can "run their lungs" longer b/c of the less resistance compared to roadwork. You see, roadwork will fatigue the muscles before the lungs due to the pulling the ground. Conversely, the mill will allow less fatigue on the muscles, but the breathing and development of BREATHING continues on.

I think there's a use for both methods and I know both forms of work are sport specific.