Like cyccommute, I wear a Camelbak on every ride, whether it is local or on tour. LIke him, on hot days, I put ice in the bag and drink cooling water for hours. I also use it to keep foods I don't want to get too hot just a bit cool, like cheese or chocolate.

I made the switch when I got a kidney stone from not drinking enough water when I rode. Now, I don't have that problem.

I sweat alot, so backpack or no my jersey gets soaked. I do find that when the full 2 liters of water are in the bladder, it feels a bit heavy. But, after a bit of riding and sipping, I don't notice it. On average, it is only half-full ( or half-empty, take your pick).

Lastly, I keep all my valuables in the backpack, so I am never without them.