Americast is American Standard's brand name for a tpye of fixture now made by several companies. The body is a molded polymer material; the surace has a very thin metal layer,over which the porcelain finish is applied. They have the rigidity of cast iron, claim to have the same durability. Supposedly do NOT chip easily like the steel tubs and sinks can. For a tub, they hold the heat much better than cast iron.
These have been around for several years. I have not heard any negative reports. Price wise, similar to cast iron. Because they weigh so much less, your back will love you for that.