I have one, and use it to keep my axe/hatchet heads sharp. They are coarse on one side, and fine grit on the other, and work quite well in my opinion. Mine has dished out a little from many years of use, but I don't see that as a problem, as the dish fits the convex profile of my heads.
Of coarse if your head is pretty dull a file would be a better choice to get it to the point the stone should take over. Once sharp though, the "puck" will keep it that way!