Enamel is baked on at the factory to the cast iron protecting the cast iron. This provides a non-reactive finish which is excellent for soups, stews and braises. It's susceptible to chips, scratches, dings and temperature shock (rare). It generates a better fond than seasoned cast iron.
Seasoned cast iron is protected by a carbon coating (from the baked in oil). This coating is not impervious as the enamel is but still somewhat reactive to acidic foods (tomatoes and red wine for longer than 10 minutes for example) or to long stewing/soaking. However, the carbon patina is fairly non-stick and can take very high heat without damage and is generally a high performance cooking surface that improves with use.
The bottom of an enameled pan is coated too, just with a different coating.
This kind of enamel is essentially a type of glass. I watched them re-finish an old cast iron bath tub on This Old House once. A similar technique is used for the cookware, just a different grade of enamel and probably more automated.
The tub was stripped of it's old enamel and heated red hot. The tub was on an arm to tilt it around to expose all the surfaces at various times. The guy applying the enamel had a special sieve on a long pole. The sieve was full of powdered glass. The glass dropped through the sieve at a given rate and the guy moved it around to apply an even coat to the tub. The glass melted and bonded to the cast iron and was fired further to create the final effect.
For cookware the bottoms and the rims are finished with a tough, slightly rough coating. This coating is more capable of standing up to the abuses these surfaces recieve that the shiny smooth enamel could not resist. All areas of the pot are treated or rust would break off the enamel coating. This also has to do with equalizing the thermal stresses on the coating.
I have a pan with orange enamel on the outside (le creuset style) and the inside is simply cast iron. I think it was given to me by my family, and I can't see any benefit of this over pure cast iron. Perhaps theses combination type skillets aren't made anymore?
I don't see any benefit either and I wonder about how durable it is at the edge. It was probably more marketing than performance. Get some color into the line and tout some reduced maintenance perhaps?
YES: Both enameled and bare cast iron have good even heating and heat retention.
NO: Bare cast iron can become quite non-stick yet still generate an acceptable fond. Enameled cast iron is not non-stick but generates a good fond
NO: Bare cast iron is a reactive metal. This means that some acidic foods may discolor and/or pick up an off metallic taste. The primary offenders are tomatoes and red wine. White wine to a lesser degree. Length of cooking time also enters in and many feel that tomatoes or wine cooked less than 10 minutes won't have that discoloration or off taste.
Similarly, the patina (the black layer of well seasoned cast iron) is SLOWLY soluble and can be removed or weakened through long wet simmers.
Enameled cast iron is coated with glass and is non reactive. This makes it a popular choice for stews where the meat is browned (good fond) and simmered-often with wine tomatoes and so on.
NO: Clean up. Well seasoned cast iron cleans fairly easily. It is often cleaned with just hot water and a stiff brush and can often just be wiped clean. It is not dishwasher safe.
Enameled cast iron, if fully clad in enamel, is dishwasher safe. Le Crueset and Lodge pans, even with the black rims, are fully clad. The black rim is a coarser matte enamel to protect the high wear areas. I suspect KYHeirloomers rims are coated or were coated at one time?
NO: Durability. While both bare and enameled cast iron are tough and durable, the enameled is subject to chips, cracks and other failures in the enamel surface. With proper care, they should both be heirlooms to be passed down.
Maybe: Many people are happy to serve elegantly at the table in the enameled cookware. I'm happy to serve out of plain cast iron or stainless steel but I may well be a boor.