Well last night I ran 9 miles with a water bottle belt, but used a different water bottle to the one the belt came with. I noticed the weight seemed off balance and after 9 miles had bad pains in my right leg, which I believe is due to having a good proportion of what would have been about 750g extra weight over the right hip. Having a full CamelPak weighing 1-1.5kg, but evenly distributed, causes no probs on 20 mile+ runs....