My biggest problem with this theory is the fact that people throughout human history have done some pretty immoral things that made the majority of the people happy. The most obvious example is slavery. This way of life for the white folk was easy living which of course made the white population very happy. White people are the majority even now, but at least they have realized that what they did to the black people was wrong and immoral, and now that policy is outlawed. Using the Greatest Happiness Principle, back then this would have been an ethical and moral act because the majority of the people were happy and society was functioning very productively well.
Slavery is however almost about as immoral as an act can get.
This is why I do not and conscioulsy cannot agree or support this theory. For any ethical or moral theory to be worthy, I think morally is has to be problem free to even be condisered a moral theory at all. According to Mill, the rational principle that determines whether an action is morally permissable is the greatest happiness principle. This principle is different than Kant's rational principle, called the categorical imperative, because this principle does not have to become a universal law in order for an action to be right or permissable. The role this principle plays in Mill's Utilitarianism is that actions are moral if it promotes happiness.