It wasn't until very recently that I realized that people eat cactuses. I always thought of them as being ornamentals, unless you were, say, stranded in the desert without food and water in a Grade B Western. Well, it turns out that I was very misinformed--lots of people eat cactus, both the stems and "leaves" (pads), and the fruit. The fruit can be made into jellies, candies, and even alcoholic drinks. I was also somewhat surprised to learn that the cactus, except for one species that's found in tropical Africa, Madagascar, and Sri Lanka, is solely a New World native. It has, though, been sucessfully introduced to many other arid areas around the world.
I started with eating part of the stem or pads. This was found in the Mexican section of a Virginia grocery store, and I've seen it since in many other stores. These inch long, about half an inch wide sections were packed in water, and are usually eaten as parts of a salad. I had them plain, and came away very unimpressed. They were almost entirely tasteless. Think of the most overcooked green beans you've ever had--extremely bland. To be fair, maybe they're okay mixed with other fruits or veggies in a salad, but I'm perfectly happy to never have them again.
Fortunately, I had much better results with a type of cactus fruit, the prickly pear. The type of cactus is comes from is evidently the cockroach or coyote of the cactus world--it's the only one which thrives in the Great Lake States, Long Island, and even parts of New England. The fruit is also nicknamed "cactus fig" or "tuna" (this last one apparently from a Spanish word, and has nothing to do with the large fish). The prickly pear is about the size of a large kiwi fruit, and is covered in thick, rough, greenish skin. Once this rind is removed the inner flesh is revealed, which is an intense blood red. They're chock full of big seeds, but these are edible, too. Some folks think they have a watermelon or even bubblegum-like flavor. For myself, I was reminded of an apple-y or melon-y flavor, with a pear-like texture. Good--I'd be willing to buy these again. One word of caution, though--if you're harvesting them yourself, make sure all the tiny hair like spines are removed, as they can cause very unpleasant mouth and throat irritation.
Therefore, to sum it up, I'd say when it comes to cacti, the stem/pads aren't worth the trouble, but the fruit is tasty. I'm looking forward to trying another cactus product, the dragon fruit.
To add one more bit of cactus trivia, to discourage defectors, in 1961 Cuba had its soldiers plant an eight mile long barrier of cactus along much of the fence surrounding Guantanamo Bay. It became known as "The Cactus Curtain."
Finally, you may have noticed that I used "cactus," "cacti," and "cactuses" as plural forms of cactus. According to the dictionary all are correct.