I shy away from magnetic knife bars due to personal experience. I had one, and some of the knifes were too heavy, or didn't make solid contact (Chicago Cutlery hollow ground), and so fell off regularly. And if you are using stainless steel, some will stick to a magnet, while others won't. Ferritic stainless steels are generally magnetic while austenitic stainless steels usually are not.
So, before purchasing a magnetic knife bar, make sure all of your knives will stick to a magnet.
Not relavant for this thread, but I saw a post where it was stated that a pan needs to be magnetic to work with an induction stove. That is not true. It does need to contain iron, however, and should be a relativley poor conductor of electrical current, thus producing heat as electrical eddy currents are produced through induction, a process where expanding and collapsing magenetic flux lines pass through the metal, creating electrical currents. Due to the resistance of the iron, or steel, the metal is heated by the eddy currents.
Seeeeeeya; Chief Longwind of the North