I do agree with Diamond's major thesis in this book. He basically argues that geographic factors are what caused some societies to become dominant and others to get dominated. He says that it is these factors which have nothing to do with human beings (and not the people's racial characteristics or their cultures) that have determined the overall course of history.
Perhaps the most persuasive anecdote that Diamond cites is the experience of the Polynesians. The fact that Polynesian people evolved so many different kinds of societies, depending on the physical characteristics of the places where they lived, seems to be a very convincing argument for the idea that it is geography that matters.