DS11 got one because all his classmates had them. As luck would have it, they were banned from school a couple days after DS11 got his. He still likes it, though, and uses it at home.
They're actually a therapy-type device for children with issues, it helps them focus. I figured I'd get one for DS20, who has Asperger's and several other issues going on. He says it makes a huge difference to him, really helps him calm himself when he's trying to talk--he has difficulty articulating. So, his doctor is writing him a note, which we'll turn in to Special Services at his college, so it's on record that he's allowed to have the device due to his issues. My younger son has no disabilities, so he's out of luck, LOL.
For most kids, it's just a fad. Remember Silly Putty? Duncan yo-yos? Tamagotchi? Beanie Babies? Slap bracelets? Garbage Pail kids? All have had their 15 minutes.