I use a Mountain Smith Day Lumbar pac and have hunted with it for over 20 years. I wouldn't trade it for anything. I can lash a jacket to the outside of it and can carry everything I need in it for a days worth of hunting.
I usually have a knife, 10 rounds of ammo, 24 ounces of water, two cliff bars, a windproof lighter, small digital camera, gloves, stocking cap, headlamp, and a road flare with me.
It hasn't failed me yet.