The original intent of the eyes was certainly to serve as advertising for long ago oculist Doctor T. J. Eckleburg. Given their location, looking out over the valley of ashes through which people pass as quickly as possible with as little time spent taking in the scenery as possible, they may have never been truly effective in attracting customers. Certainly, in their state at the time of The Great Gatsby, "dimmed a little by many paintless days under sun and rain," their function as a symbol of consumerism is probably not as significant as is their possible symbolism of God watching the events taking place.
Perhaps the strongest symbolism to be associated with the eyes reflects the complete lack of any morality or compassion for others. The eyes are witnesses to the corruption of character, the materialism gone mad, the conspicuous consumption that dominated the lifestyle of the Buchanans and that Gatsby sought to achieve as part of his American Dream.