I work in Saguaro National Park as an intrepretive naturalist during the winter months. The fruit of the Prickly Pear, the Saguaro and even the Barrel cactus are edible. The Prickly Pear and the Saguaro fruit are very tasty, the barrel cactus fruit - not so much. Small spines on the outside of the Prickly Pear fruit need to be removed before consumption. Consuming the liquid inside of a cactus is only to be done as a last resort since many of the members of the cactus family concentrate alkaloids which are not good for the human digestive or the human kidneys. The pads of the Prickly Pear cactus are edible but not all that tasty, from my perspective. The best bet when traveling in the desert is to carry an ample supply of water, and admire the cactus from a safe distance.