The better milling vises (Kurt comes to mind) use the angle-lok principle on the movable jaw. This forces the jaw down onto the bed of the vise as you tighten it up, rather than lifting off said bed as a cheap vise jaw will do. When lift off occurs, it tends to roll the part up off the base of the vise or off the parallels, and you lose thickness control and squareness. You can hammer down the work all day in one of these cheap vises and it still won't sit down on both parallels. :|
If you are using this vise on a cnc machine, you don't really need a swivel base for it. I know I've got three swivel bases sitting unused in a cupboard
Sometimes it is really nice to have a horizontal swivel axis (a 3-way vise), but this type of vise is really high, and gobbles up even more of your available work envelope. Wait until you really need one of these before you get it.