you can get just a camelpack, that are usualy insulated and hold only the water, or you can get a packpack that has a slot designed for a water resevoir bag (camelpack) and may, or maynot come with it. I have a 1.5 camelpack which i like anything bigger and it would be a little to heavy for my back. The insulated ones don't heat your pack, they cool it, although you will notice more sweat just cause there's no air flow under between your back and the pack so it just can't evaporate like the rest of your back.

as for the camelpack's on their own, they're pretty good, bobby julich used one in the TDF TT yesterday under his suite. Some of the hose ends can be kinda crappy and leak. I guess it depends on the ride, but i go back and forth between my camelpack and 1 or 2 bottles on the bike. The camel pack definitly keeps water cooler longer though.