I use a Jointer Pal instead of the MagnaSet, but they're basically the same tool. They have magnets embedded in the arms that keep the jig flush to the table, and magnets at the ends of the arms to hold the knife being installed flush against the jig. Some are plastic, some are metal. Mine is metal and came with my Grizzly jointer, but if I were buying I'd probably get the metal one for the reason Charles mentioned. One thing to realize about this tool is that even though the magnets are strong, they won't prevent the small amount of shifting that occurs when the gibs are tightened. To confirm this, position a dial indicator on a magnetic base over the cutting circle and rotate the cutterhead so each knife passes under the dial indicator at the same spot. Also test it along the length of the knife. I use additional magnetic hold-downs to help keep my jig in place, and it helps. Tightening the gib bolts a little at a time and in the proper order also helps. Tighten enough that the knives around flying around the shop, but don't over-tighten to the point that the cutterhead is distorted...that also helps. Once you've fussed with it enough that you know what works you should be able to get it within a thousandths all around. This is the best tool I've used for getting jointer knives aligned.
Cheers.