Objectively there is no reason why you shouldn't be making actions that server you in the best way possible. If you exclude all other factors such as morality and ethics (which are man-concepts, that have no solid logical backing), there is no reason why we shouldn't do only the best things for us. After all our natural instinct is self-preservation. Besides, if utilitarianism was implemented correctly into society, it would be highly beneficial.
As someone who has followed this school of thought for oh so many different years, I fail to see any reason why anyone would every consider it to be something that is not objectively valid. It is a way of thinking, and everyone is free to do such a thing.
I believe that utilitarianism is a method that can be used to produce reasonable results. You can produce ethical results using a measurement of happiness which can be provided by weighing utilitarianism ideals. You are able to compare alternatives which can bring you to ethical choices. You are able to measure the consequences of acts and this can help achieve the most moral decisions.
It at least keeps people busy pursuing things that give them purpose. The problem is many people's morals vastly differ. Which would not be a problem, except many people's morals conflict, and impact other people's lives in a negative way. Which in turn makes people unhappy, and very full of spite . Until people learn how to be objective on other people's beliefs, then utilitarianism will always have it flaws.
If you try to base all your decisions on maximizing happiness it self-defeats. You will not be happy if you are obsessed with going after happiness, whether for yourself or others. One should focus on human flourishing of self and others and welcome happiness as a byproduct. In fact this is more likely to work to gain you happiness.