The Introduction is a series of lectures that Heidegger presented to students in 1935; it was first published in 1953. Although several of Heidegger's other works—Being and Time, Nietzsche, and other lecture series—are, I believe, either more important or more useful, the Introduction surely is significant. This is especially so for students of politics, because in it Heidegger briefly discusses the polis, values, the "ought," and the Nazis.
Near the end of the Introduction, while criticizing the notion of values, Heidegger writes: "In particular, what is peddled about nowadays as the philosophy of National Socialism, but which has not the least to do with the inner truth and greatness of this movement [namely, the encounter between global technology and modern humanity] is fishing in these troubled waters of 'values' and 'totalities.'" Heidegger claimed to have written but not delivered the bracketed words in 1935, but he seems in fact to have first added the clarifying phrase in 1953. The reason for Heidegger's lie is that the earlier a phrase qualifying his support of the Nazis appears to have been written, the better he looks. But the lie hardly seems worth the trouble, because to praise the inner truth of such monstrosity is itself always monstrous. At the least, Heidegger's qualifying phrase makes clear that he did not believe that his seeing something "great" in the Nazis was an odd personal or political mistake. Rather, it is connected to the heart of his thinking. For the questions of technology and "truth" are vital to Heidegger's understanding and are not mere throwaway terms.
The core of the Introduction is Heidegger's remarkable discussion of physics, logos, noein, eidos, and other central elements of ancient, especially pre-Socratic, thought. Any serious student who believes himself to be oriented by Aristotelian political science, "natural" law, or Plato's ideas must come to grips with Heidegger's powerful analyses. They are similar in several respects to later discussions among students of political philosophy for which Heidegger helped pave the way; yet, for all the similarity, there are radical differences.
This is not the occasion to probe and explicate these differences or to discuss the limits of Heidegger's analysis. Suffice it to say, that one wonders whether, given Heidegger's discussion, the true limits and unity of political life can become clear, the path from practice to explicit philosophy can become evident, or the link between the everyday world and the world of what Heidegger calls the "creators" can fully be understood. Ancient political philosophy is as significant an alternative to Heidegger as he is to it.