Permethrin is not a repellent but an insecticide. However, I've found it does keep mosquitoes from biting through my clothing. It does not, however, do anything to biting flies.
Important: Carefully follow all label precautions (as should be done with any such compound). While the military does use it lavishly, permethrin is not harmless. In the case of the military, the alternatives (malaria, dengue fever, etc., etc.) are far more harmful than possible effects from the permethrin. That may not be true for us North American backpackers. I use the lowest concentration available (usually Repel Permanone), spray outdoors and upwind of what I'm spraying and make sure the clothing is completely dry before handling. I find the results just as good as with a heavier concentration.