Hell if I had one of these in school I would have not used so much paper drawing random bits of stuff or things from video games I had played in the middle of math class as the teacher droned on and on and on....
As to if they are good or bad...really its all on the person, I see nothing wrong with them, if the person is able to perform better with it then go for it, but if its too much of a distraction then its not good.
Yeah there are a lot of them out in the market now, some better than others and some are total trash, my worry is someone getting too hot under the collar and whipping a metal spinner across the room and chipping someones tooth or possible eye damage from impact.