Actually, cast-iron pots will heat less evenly than aluminum pots, for instance, since iron is a poorer conductor of heat. Furthermore, compared to iron, aluminum also has a higher specific heat capacity -- it takes more energy to raise a unit mass of aluminum by one degree Celsius than iron. However, iron is more dense than many of its competitors. What does this mean? Well density = mass/volume. So if we compared, for example, aluminum and iron pans of the same volume, the iron pan would have more mass (is heavier). So far so good, right? "OK, great, why do we care about that?"

This means that even though iron has a lower specific heat capacity than aluminum, it can still store more thermal energy in a pan of the same volume at the same temperature. How? Q = m c ∆T, where Q is the heat energy, m is the mass, c is the specific heat capacity, and ∆T is the change in temperature. If m is a lot higher for iron, then Q will also be a lot higher for the same ∆T as compared with aluminum.

Let's think about what this means. An iron pot will take longer to heat up to a certain temperature than an aluminum pot, but will store more thermal energy than the aluminum pot at the same temperature. So once it gets to that temperature, it has more thermal energy to transfer to the food we are trying to cook!