I always use mine. I am DVC so I have access to a freezer in my room. I fill the water bladder about 1.2 full the night before - freeze it solid. This way when i add water to it in the morning I have a nice cool pack on my back. I also carry my rain gear and sunscreen and other stuff that I use sprodaically during the day in the pack. I carry a small belt pack for the room key and camera - yes, I have to go through the "search" line on entering the park, but I would have to anyway because of the belt pack. To me the availability of frozen Dasani water is very nice. I don't like the taste of the water fountain water in florida.
I also find that I use the pack to carry small purchases, and I keep some high protein snacks for my hunny who is hypoglycemic. This gives us the availabilty of good snacks and keeps her sugar stable.