Very expensive but they do work as it says on the tin I have one and it does sharpen well with very little taken off but at the small end its a bit fiddly
What it won't do is split point but it will thin the point
As I use dormer drills at a price it does earn its keep but I doubt if it will repay itself , but if you have a job on and the bit is dull then it's a god send
Like the others I used to hand grind I still do for 12mm up with a chalked bar to get the lips the same but as the lamps have gone and glasses are getting thicker it works for me
So expensive , unlikely to pay for itself , useful yes
Stuart