I need to drink a LOT (due to no longer having a large intestine). So on long rides I use a 3-liter hydration pack with water and 2 large bottles of sports drink. I sip from the hydration pack constantly, then from the bottles any time I stop, however briefly. I can probably get through 2 full packs plus 2-4 bottles in a long day's ride. If I were using only bottles I wouldn't drink enough because, though I can do it, I don't feel all that confident at swiping a bottle up off the frame while riding. So for me, no choice. But yes, you have to be careful choosing a pack to avoid chafing, and yes it adds top weight and reduces the cooling effect of evaporation off your back. (btw, Trek, the pack worked fine on Saturday's ride -- no kinks, no drips, kept the water cool enough.)