I have this one:
http://www.rei.com/online/store/Prod...cat=REI_SEARCH
I've biked without it, but found that I drink too little when drinking involves stopping to pull a bottle out, or pulling it out while riding (makes me a bit nervous). I put plain water in the pack and rehydration mix in a couple of bottles, drinking from those whenever I stop. That seams to work fine. And the pack has space for me to stuff in my arm warmers, sweater etc. as the day warms up, rain gear in case those clouds burst, camera, a couple of energy bars ... that kind of stuff.