This paper discusses comprehensively the weaknesses and merits of utilitarianism. These weaknesses may be classified into two kinds: One kind is the objections to utilitarianism raised by non-utilitarian philosophers. This paper, from the viewpoint of my unified utilitarian theory (UUT), argues that these weaknesses are either derived from a misunderstanding of utilitarianism, or being concerned with only one particular form of utilitarianism, rather than a general weakness of all forms of utilitarianism. The second kind of weakness is the difficulties felt by some economists, which can be removed or proves to be actually nonexistent in UUT. This paper, from the sources of the weaknesses, also reclassifies the weaknesses into three categories: namely, difficulties arising from the utility theory, difficulties arising from the theory of value, and difficulties arising from utilitarianism as an ethical theory. From the viewpoint of UUT, these three categories of weaknesses can all be removed. Further, this paper explains three features of the unified utilitarian theory: features of my modified VNM expected utility theory, features of my subjectivistic theory of value, and features of the unified utility theory as an ethical theory. Moreover, nine merits of utilitarianism are listed and explained, seven of which are common to all forms of utilitarianism, and the remaining two are particulars to UUT. Finally, this paper proposes a revised principle of utility, which is the same as that of act utilitarianism, except for three additional provisos.