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.