Cooking surface: 2/5 Poor (3/5 Good once seasoned, it is less sticky as the surface becomes like hard plastic; but still struggles with acidic foods so it’s not for all-purpose cooking)
Conductive layer: 2/5 Poor
External surface: 1/5 Very Poor (3/5 Good if seasoned on the outside as well as the inside of the pan, to make it more rust-resistant)
Example: Lodge, Staub/Le Creuset (for enameled cast iron)
Health safety: 4/5 Good (assuming no ill effects from seasoning)
First, the pros: cast iron is cheap and can develop a semi-nonstick surface (“seasoning“) and can be heated to very high temperatures safely, and going from stove to oven (or vice versa) is one way to work around cast iron’s relatively poor thermal conductivity. Cast iron is usually made 3mm+ thick so it will have excellent heat retention and not drop in temperature too much even if you throw a cold steak on it, but note that the same can be said of things such as stainless steel with thick aluminum disc bases, which do not cost much more than cast iron. Thick cladded designs like Demeyere’s Proline skillets (4.8mm thick, 3.7mm of which is aluminum) also have great heat retention. Pound for pound, aluminum has nearly double the heat capacity of cast iron and can recover from losing heat much faster and more evenly.
Cast iron has a lot of cons, too. Cast iron is heavy, which presents safety hazards for some people (e.g., cast iron on a high shelf). Cast iron handles also get hot, which can present a safety hazard for pets and kids who don’t know better. Cast iron is also somewhat brittle (do not bang it on hard surfaces or heat too quickly), does not heat evenly on stovetops (everything, even glass, heats evenly in an oven, so that doesn’t count), requires more upkeep than stainless steel and aluminum, and in the case of enameled cast iron, enamel can chip or wear off over time, particularly if it’s lower-quality enamel. Cast iron is usually made thick and heavy, with large heat capacity in the metal, so it may take a long time to heat up. This means cast iron is not very energy-efficient, but at least it can be used with induction cooktops which are energy-efficient.
Cast iron cookware is made from pouring molten iron into casts and machining the result. Cast iron is naturally magnetic and works on induction. However, cast iron is also brittle and can warp, so it is usually made quite thick (>3mm). This means cast iron cookware must be heavy, which prevents its use by some people. Cast iron can also rust when exposed to too much moisture. Bare cast iron can also leach iron into foods, though usually not enough to be a serious health concern unless you are male or post-menopausal female and cook a LOT with bare cast iron. A less-serious but more common problem with iron leaching is off-flavors: acidic foods, such as tomato sauce, leach so much iron into the food that you can get unappetizing off-flavors as a result. The more seasoned a cast iron pan is, the less iron leaching, but there will still be some leaching.
To combat the drawbacks of cast iron leaching and rusting, cast iron is sometimes coated with enamel (Staub, Le Creuset, etc.). The enamel coating leaves only the rims exposed (so you may want to season the rims), but the rest of the cast iron cookware is protected from rust and from leaching iron into food. However, enamel is brittle and large thermal or physical shocks can chip or crack enamel, especially poorer-quality enamel. (I once bought a Better Homes and Garden enameled cast iron dutch oven once that developed very small but noticeable microfractures after just a few uses.)
Please see Seasoning for more information on how to season your cast iron. Cast iron and carbon steel have lots of iron that will oxidize (rust), especially if left in contact with water. Furthermore, seasoning may come off if you use detergents, which act as chemical go-betweens for oil and water. Therefore, never leave your cast iron/carbon steel in water. It’s okay to wash with hot water, then immediately towel dry and then heat up briefly on the stovetop to get rid of any traces of water left. It’s also okay to simply wipe with a paper towel and perhaps some salt, if that helps get rid of any charred bits stuck to your seasoned pan. Note that cast iron is oven safe, and so is seasoning since it’s already basically burned-on oil in the first place.
Enameled cookware is tougher. In fact, enameled dutch ovens are the classic one-pot-meal factories: brown meat, saute onions, etc. and put the rest of the stew inside. Heat it. Eat it. You can even refrigerate the leftovers in the same pot–you don’t need to put leftovers into Pyrex containers or whatever, because enamel is immune to acids and salts. Storing acidic or salty food in other cookware is usually a great way to corrode and ruin your cookware. Not so with enamel-lined cookware. Just be careful not to bump your enameled cast iron into hard surfaces, as that may chip/ding/damage the enamel. Also note that enamel varies in quality and thickness, and the market leaders Le Creuset and Staub/Fontignac have the best reputations for long-lasting enamel. Lodge and Tramontina are other reputable brands, even though they are made in China. Costco has a contract with Zwilling, the parent company of Staub and Fontignac, so if you see Kirkland Signature enameled cast iron that says it is “Made in France,” it’s probably made by Staub or Fontignac. (Some claim that the Costco enamel is thinner or more poorly applied. I haven’t been able to tell a difference in thickness, but I do agree that the quality control of Costco enameled cast iron is poorer, with more missed spots or thin spots.)
You should not heat any type of cast iron (bare or enameled) on high heat. Cast iron is brittle, and so is enamel, so sudden temperature shocks may crack the cast iron or enamel.
Well-seasoned cast iron is semi-nonstick, making it a favorite for cooking things like eggs and fish. However, cast iron does not heat evenly, so you may want to do things such as preheat the cast iron in an oven or very slowly over a stovetop in an attempt to make it more evenly heated before putting food on it. (But note that the cast iron may become unevenly heated again once you put food in it, or even if you just leave it sitting on a burner, because the heat coming from the burner needs to replace the heat lost to the food, and cast iron is a relatively poor heat conductor.) Cast iron tends to produce lovely fond despite its semi-nonstick nature, though the dark color of seasoning makes it a little harder to see the fond.
Please note that if you like the idea of seasoned cast iron but want less weight, you can use carbon steel instead. For cooking purposes, carbon steel is basically the same as cast iron, but can be made thinner and lighter than cast iron. Both are poor heat conductors, though. Virtually all carbon steel cookware on the market is thinner than cast iron, thus they are even less even heating. At least they weigh less.
Enameled cast iron is like a stickier version of cast iron, but it cleans up more easily. Staub uses slightly rough black enamel that they claim can thus take on a little bit of seasoning over time.
Please also see the benefits and drawbacks of seasoning.