Pro: The rough surface is coated with bits of diamond, which are much harder than the steel in the knife blade, so it sharpens quickly. Professionals prefer using a diamond steel to machine sharpening, which can take off too much metal.
Con: Using this requires skill or you may dull the knife further.
Pro: Purists swear by porcelain because it creates a fine edge without removing too much metal. This one works by drawing the blade straight down the side of the cone, so you needn't worry about maintaining the proper angle.
Con: It takes up a lot of real estate in a cabinet.
To buy: Unfortunately, this product is no longer available.