I bought one for my daughter. She's 24. However, she has many, many medical problems (is medically disabled) and has anxiety and depression. One thing that she does is pick her fingernails (and I mean to the point where they bleed). This spinner, although it can be annoying, is helping distract her from picking, and she showed me she actually has a small amount of healthy nail growth on her fingers.
I was happy that I didn't have to worry about classrooms, etc, with her. She lays in bed when in a flare-up from her various medical conditions, watching a movie on her iPad, and that's when she used to pick her nails to the point of getting them infected. Now she spins that thing and can't pick.
So, I'm kind of glad the things exist, although my friend's daughter is a teacher and she's having lots of problems setting boundaries with the spinners. They were meant to be a tool for helping with concentration, focus, etc., but it seems, from what I've heard, that all the kids can have them in class. They're like a legal toy, and in many cases they're causing the opposite effect of how they were intended (they're distracting, instead of helping with focus). I see them being quite limited in classrooms in the near future. Maybe kids with concentration issues would be allowed a "spinning" break in a separate area, etc.