"Nopales" or "nopalitos" are actually the flat green pads. They have a vaguely green bean type of flavor and are a bit slimy inside. I dice them up and add them to my salsa. Good stuff.
The prickly pear fruit (technically known as tunas) bud along the edges of the pads and turn bright-to-dark red when ripe, depending on the local population. Sweetness and acidity vary from region to region, as does strength of flavor. I've tried them off and on and they can be lemon sour, syrupy sweet or mild. The flavor is vaguely melonish, familiar yet hard to place. Others have compared it to dragonfruit. They grow all over the place here and will start ripening at the end of the month, so I plan on doing some harvesting.
Mexican groceries will sometimes have them. The regular supermarkets will have them around here sometime, but they're always green. I don't understand that, since the fruit won't ripen once it's picked. If you get them from the store, they ought to already have the spines removed--the fruit have tiny, hairlike spines in tufts here and there, that are awful if you get them stuck in your skin. I recommend gloves when handling. Some singe them off with an open flame, as the spines burn easily.
I always freeze my pears, as it liberates the juice and makes preparation (at least for me) easier, and aids in liberating the juice by rupturing cell walls. Wearing leather gloves, I'll chop the ends off then slice the skin down the middle and peel it off, placing the seedy pulp inside off in a bowl or somesuch. They are VERY seedy. The juice is deep maroon and will stain. I've heard others blanch the fruit for removal of the skins, but as the juice gets everywhere even when frozen, I can't imagine how sloppy messing with thawed fruit would be.
It's my understanding that the fruit need to be simmered for 30 minutes before adding to the fermentation vessel, otherwise it will foam up like the apocalypse. Not wishing to test this theory, I always simmer. Some say to boil, but as that would set the fruit pectin, I keep the temps below boiling. I mash the fruit in the pot to extract as much juice as I can, then strain it all and add to the secondary fermenter. Disappointingly, what starts out as an incredibly rich burgundy-maroon color steadily fades during aging until it's merely a soft pink blush in the mead. The flavor's delicate and hard to place--I was greatly amused by giving folks a taste and their inability to guess what fruit they were tasting.
For a six-gallon batch of sweet mead a few years back, I used 18 pounds of clover honey and 15+ pounds of prickly pears. At the time I pitched Pasteur champagne yeast because I didn't know there were better choices. I'm going to up the ratio of fruit per gallon this time around, pick a different yeast more suited to melomels, and ease back on the amount of honey, back sweetening if necessary. That first attempt was too sweet (yes, even with the Pasteur), and would've benefited from a more generous addition of tannins. I blogged about the process: