I've used a number of diamond stones n the past, including Swix, DMT and some cheap nameless ones.
They did not cut NEARLY as fast as the moonflex stones.I did not get the black, just the red, yellow and white. After a bit of usage, I'm happy with that combo as the red cuts so fast I can't really imagine what I would use the black for.
I can't yet speak for the durability, but the careful and clear directions for use (keep it wet and clean)are very good. As far as use on the slope, I just spit on the face of the stone and wipe it clean on the small cloth I wrap it in (the manufacturer suggests using snow, but I find it freezes my already cold glove less hands
and doesn't work as well).