DH and i took one last year and we plan on taking one again this year. i'd put about 3-4 bottles of water in the fridge at night and then we'd fill up the bladder with them in the morning before we'd go to the park. we shared so we'd run out of water by around lunchtime, so we'd just fill it back up at a water fountain with cold water and be good to go again! worked great!!! the only part of the water that got warm was the part that sat in the straw, so DH had the job of sucking that out and "getting rid" of it, then the cold water would come out. better than spending $3 a pop for a bottle of water!
we were sitting in tomorrowland sipping out of it and eating a spicey cheese pretzel when a young male CM came up to us and asked what "that thing" was. we told him it was a camel pack and that we were drinking water out of it. he then asked if security had checked to make sure it was water. we told him no, they didn't even ask and he said, "wow....so you could bring in any liquid in that, huh? good idea!" so if you see a young male CM in tommorrowland wearing a camelpack and acting a little loopy, don't blame me that he filled his bladder with vodka!