I used to use one a long time ago, and they are ok for "mass feeding", meaning when you have a ton of dither fish who aren't afraid of each other and all go up for their share. However, food is gonna sink, it's just gravity. It doesn't stay in that little circle for long. If you have a really strong "current" from filters/airstones/powerheads, then it helps for those fish who can't or don't want to swim to follow the food as it descends into the tank, other than that they are more gimmicky than useful, as the food will sink.
HTH