If an unlined aluminum vessel is used to cook a high-alkali food such as potatoes, or if the cooking medium is hard water, or if the pot is washed with high-alkali cleanser, the metal's surface becomes stained. When the pot is subsequently used to cook tomato sauce or any other high-acid ingredient, such as onions, wine, lemon juice, or cabbage, the acid chemically removes some of the stain from the pot and transfers the discoloration to the food. Although the brownish tinge diminishes the aesthetic appeal of the food, it should pose no threat to your health.
Another drawback of aluminum pots is their propensity to warp when subjected to abrupt changes in temperature extremes (more so than, say, stainless steel of identical gauge). And aluminum implements dent easily, especially if they are thin-gauged.
On the plus side, the heat-flow efficiency of a thick-gauge aluminum pot nearly rivals that of a copper pot of similar gauge, which is noticeably heavier and many times more expensive. Unlike cast iron or carbon steel, aluminum doesn't rust (though it does oxidize slowly). If treated with care, aluminum pots will last for decades.