Masahiro is known for the high quality of their carbon steel blades. Carbon steel takes an extremely fine edge very quickly. The trade-off for this feature is that it lacks the "no stain" advantage of a stainless steel blade.
Virgin carbon knives take a sharper edge than any stainless steel, and are easier to resharpen. The edge breaks down faster than stainless if used on highly acidic foods like citrus. A patina will form with a couple weeks of usage which retards rusting. If you cut a lemon and forget to wipe your knife, just clean up the rusting with a little BarKeepers Friend, available at most grocery stores.