i have a hydrapak reyes which has a 100oz bladder. i usually fill it about 3/4 of the way up.

i know it's not a super popular choice to wear a hydration pack as a road rider but i really like it a lot, it's very practical for me as a city rider. i bought my specific pack because it is a bit bigger, so i can carry some stuff in there too. i throw my (fairly large) chain, pump, lights and a little bag that contains my multitool, a spare tube, etc. in the main compartment, and i have room to spare for some snacks, wallet/keys/cell, and an extra shirt (or whatever) as needed.

i adjust what's in it by the ride... for a shorter ride, i can remove the bladder and just use it as a small backpack if I want. for organized events (such as centuries) i ditch the chain/heavier crap and use it pretty much just for hydration and to hold some snacks.

as mentioned above, water stays pretty cold in it and i actually love that the hydrapak has a really easy filling mechanism where the top opens up completely, so it's easy to fill with water and/or ice. i can stop at a rest stop or store and just throw a big chunk of ice in it.