Is the United Nations useless?

This is the question I had to ask myself after my course about International Organizations. The teacher is supposed to learn us how these organizations work and what they are actually doing but it turned into a harsh criticism of the United Nations as a useless organization. The teacher pointed out that it has permitted the avoidance of another Global War but he was seemingly convinced that, except for the symbolism of the institution, it was just pointless.
I just had a problem with that. It is true that the United Nations just sucked in a lot of conflicts. The last one was the Iraq War. The USA just refused to let the organization determine what they were supposed to do. It is also true that the UN troops are just sometimes easy targets for terrorists and rebels as they don't have any power.
However, these organizations are only what the States want them to become. It is indeed true that the UN should be reformed from an institutional point of view BUT if it does not work, it is only because countries such as the USA just do no respect the obligations written in the Charter of the UN. When they ratified the Treaty which created the UN, the USA acknowledged that they would respect the written duties of the Charter. Pb: they only respect that text when it advantages them. The problem is also that no country is courageous enough to say fuck to the USA, the only superpower of the world right now. One solution for some people is that we just have to wait for the appearance of other superpowers such as China, India or the European Union. But right now, it's the shit and the image of the UN does not deserve this destiny.
The UN is actually doing a lot through their different agencies such as the World Trade Organization that obliged for example the USA to stop its taxes upon importations of European iron (or coal?). The UNESCO and the UNICEF are other examples of successful organizations of the UN. I do believe that the world is better with the United Nations.