I have two.
The 100 oz Camelpak Lobo(wow, this is a great price!)
http://www.performancebike.com/bikes...=camelpak+lobo
Plenty of room, but 100 oz is too heavy in an agressive riding position. It pushes you down. Could hurt your back too. But this model allows more air circulation under the harness, and is quite cool for a pack this size.
Also have a 50 oz Camelpak. It has about enough storage for keys and an ID. Little else. Also, the cheaper models cover a large area on the back, and you sweat more. Definitely need to refill it if I'm out for hours in this heat. Even with spare bottles.
New model of the 50 oz. Mine only has one zip. Might be good for someone on a shorter ride:
http://www.nashbar.com/bikes//Produc...2_513423_-1___
What I need to get is a 70 ounce. Something that works better in an agressive position, but provides adequate storage and enough water to last.
So, I recommend a 70 ounce pak, one with a slim enough width to provide more ventilation to the rider, plus a few slim pockets.
Edit: another 100 oz pack, with more pockets. It's supposed to be good. Just remember how heavy 100 oz of water is. If you ride mostly upright, that might be okay though. Still, this pack would be warmer than the first 100 oz pack I mentioned.
http://www.performancebike.com/bikes...551_1059997_-1