Contrary opinion: From a behavioral standpoint, with a properly setup Camelback, you are likely to drink more and stay better hydrated, particularly in hot dry climates.
+1 And, since a water pack is usually insulated, you can fill it with water AND ICE, giving you cold water to drink for hours at a time, as well as keeping your back cool. Water bottles, having less volume per, and usually not being insulated will not keep your water cool for as long. I often wonder if those who disparage water pack usage (or wide, fast tires or bar bags a la francaise, etc) have ever actually TRIED the things they dismiss so easily...
SP
Bend, OR