Camelbak has a number of models, some are to carry the bladder only, and some have varying degrees of capacity for other gear. There are lots of other companies making hydration systems now, and hydration-system-ready packs; you can assemble about any degree of capability you need.

I use a "Packteen" inside another bag for cycling, and I have a CamelBak H.A.W.G for hiking... the latter comes with a 100oz reservior, and will accomodate a second reservoir (for 6 quarts) plus gear. That's more water than I'd ever want to carry in it.

The convenience on the trail (or on bike) does make a huge difference. Since you no longer have to stop or pause to drink, the low-levels of thirst that we've all learned to ignore during exercise just go away, and both speed and enjoyment increase.