I've got one, and it works pretty well for me, but I'm no master bladesmith. I do check it with a square almost every time I use it.
I use it as a rest for the blade as I'm grinding using a push stick to apply pressure to the blade against the platen as I pull or push the blade across the belt. IMO it allows me to focus on the angle of the blade against the platen and takes away a variable that I'd have to pay attention to otherwise, like having the belt pull the blade down the platen causing the blade to twist. I did grind a notch in the work rest so that it wraps around the platen to prevent the blade from getting pulled down the belt and jamming between the platen and the table.
Another way I use it is for rough shaping of handles. I made a jig with a piece of 2" angle iron welded to a flat piece that I clamp to the table allowing me to hold the handle block at a consistent 45 degree angle to the platen. The main thing that I don't like about it is that the design makes is that the design doesn't allow much room for c-clamps to hold the jig to the table on the side where the arm is. If I could redesign it, I'd move the L-shaped piece where the name plate is to the very outside of the arm and weld the 2 pieces that hold the ball on the top instead of the side of that piece to maximize the amount of room under the table.
I hope my explanation is not as confusing to y'all as it seems to me.
~billyO