It is worth noting that the green pads can be eaten, and are part of a few
Mexican dishes (
nopalitos). Although I've never tried eating the leaves, they can be bought in
supermarkets (even Fry's!) here in
Arizona. Why anyone would buy them when they are so readily available is beyond me. While you don't see them much in the cities, there are lots in the
outskirts, in peoples'
yards, or in forgotten pockets of wilderness, like alleys and untended street
medians.
Prickly pear cacti can grow very large -- there was a single plant (or community of plants) on the other side of the fence from my elementary school playground that took up around 25 square feet and rose to a height of around 6 feet. Tasty.
The fruit, when ripe, is purple-red in color and can be made into a tasty jelly, but the preparations are time-consuming and prickly.