My smith uses them. He has a couple of them. He's having Hornady make one for him for his .300 Ackley, and told me I could use it when I built one, for fire-forming my .300 Wby brass into .300 Ackley without wasting powder or bullets.
Seems like a great concept, but I haven't actually used one yet.
They are worth it if you have an overbore wild cat and are worried about barrel life when fire forming 100 + cases. The cost is why most people don't buy them but they do work.