On the list of activities I'd rather not do for fun, eating a cactus is somewhere between slamming a door on my dick and getting a face tattoo. That's just me, though. There are a small number of people on YouTube who have taken to eating cactus simply to be members of the "the Cactus Challenge" club. It's a bit like the Ice Bucket challenge, except it serves no purpose other than showing the rest of the world that you did something kinda stupid.
While eating the cactus might seem like the worst part, remember, those cactus needles are going to make their way out of one's body the next day.