I've heard both good and bad concerning National, but haven't tried it myself. However, it's important to remember that when changing foods, it sometimes takes several weeks for some dogs to adjust to it. But, it could also be that it just doesn't agree with the dogs. Keep in mind that other things could be causing it too, not just the food. Not sure what it is, but it does cause diarrhea, and it can go unnoticed.
Antibiotics seem to clear it up though, according to another musher.
I've tried a couple of different foods and settled on the First Mate. I know that if I felt something wasn't right after a while, then I stop feeding that particular food.