I have seen a number of different self centering vises out, there, but have not had the chance to play with any of them yet. How good are they? How consistently will they position a work piece?
With a regular vise the fixed jaw is pretty much always in the same place if the vise is bolted down, but there is no fixed jaw on one of these.
I can see some obvious uses in 4 axis machining, by pairing them up. They would be able to do some things say on a driven rotary table paired with an "idler" rotary table that would be pretty hard to do with a tombstone or a lathe chuck. It would only be worth the effort if they very consistently positioned the work piece however.