I use them, as I work with glass composites a lot, which will wreck normal files very quickly. They're not cheap, but they do work well.
I find the coarse ones most useful, then do final finishing with papers, my most used Permagrit tool is a half round coarse file. They also do thin shim with the Permagrit tungsten carbide bonded on, which you can in turn bond onto blocks to make custom sanding tools. These are best value compared with the premade sanding blocks they sell.
The fine grades I find less useful, as you don't get such an efficient cutting action.
If you're working mainly with metals, unreinforced plastics and wood, then the benefits are much lower, and normal abrasives are probably just as good.