I like the 100oz (3 liter) Camelback system so much that I purchased the bladder/insulated cover kit to stow in whichever of my backpacks I'm carrying when my day pack (Camelback Rim Runner) is not big enough for the outing. Then I added the insulated hose kit to THAT as well...

I still carry at least one polycarbonate Nalgene bottle or a US Canteen, but the first time I used my hydration pack, I was CONVERTED. 'Nuff said.