Yup, night before a ride in Summer I'd fill the bladder quarter full, lay it flat in the freezer, next morning top it up and have ice cold water for the next couple of hours.

I don't really care for wearing a full camelback when on the road bike. I think maybe when I'm on the drops, the entire pack is resting flat on the back and tends to sway a bit when cornering. I also do not feel the need to carry too much stuff, just a spare tube, C02 and microtool, with some cash (coins for payphone) just in case.

When mountain biking I have a 100oz Mule and enough junk to fix any mechanicals and a comprehensive first-aid kit. I've also lost water bottles mid-ride and no way I could grab a bottle on the move like I do on the road. I'm more upright on the mountain bike so most of the weight is bearing on the shoulders and waist.