Thus Spoke Zarathustra is one of the strangest books in the Western philosophical tradition. It is a mock-gospel: it relates the sayings and doings of Zarathustra in a style reminiscent of the Gospels in the Bible and it is laden with biblical allusions, but it also harshly condemns Christianity and mocks the idea of a holy scripture or a holy person. Zarathustra is essentially a man who praises laughter, and who is able even to laugh at himself.
That being said, the book is also extremely uneven. Nietzsche wrote it in ten- day bursts of inspiration, and it is clear that he didn't revise his work very carefully. The book is longer than it needs to be, and is often self-indulgent and clumsy. Nietzsche seems frequently uncertain as to what degree he wishes to engage in allegory and symbolism and to what degree he wishes simply to make a point. However, at its best, Zarathustra is unquestionably a masterpiece.
Nietzsche's subtitle—"A Book for None and All"— might help us to understand the peculiar style in which it was written. Nietzsche was an incredibly lonely man, and believed, quite rightly, that none of his contemporaries understood him intellectually. He knew perfectly well that his works would be misunderstood, and his writings are replete with harsh condemnations of "the rabble." In that sense, Zarathustra is a book for none: Nietzsche feared that his writings would fall on deaf ears. On the other hand, his subject matter concerns the fate and destiny of the human race, and in that sense it is surely a book for all. The fact that Nietzsche felt his work to be of supreme significance coupled with the fact that he had no sense of an audience might explain the crazed audacity of his writing. The best model for his purposes would be hagiography or religious scripture. The only difference is that he needed to lace his writing with laughter and irony that would mystify solemn thinkers.
We can approach Nietzsche's philosophy as a whole, and Zarathustra in particular, by grasping the principle of the will to power as the fundamental drive of all things. Everything must obey something, and if one can't obey oneself, one must obey someone else. True freedom is only granted to those who can command themselves. The will to power does not apply only to beings, but also to ideas: religion, morality, truth, and other concepts are all subject to the same struggle for power that dominates life. Because all things are characterized by a constant struggling, striving, and overcoming, nothing can remain fixed in place for too long. All things are constantly changing; permanence and fixity are mere illusions.
Most of Nietzsche's likes and dislikes, and his higher concepts of the overman and the eternal recurrence, all follow from the principle of the will to power and the attendant principle that everything is in a state of change. For instance, Christianity's belief in absolutes or in God, the rabble's love of nationalism and democracy, the scholar's obsession with truth, can all be condemned as contrary to the spirit of change, impermanence, and inequality that are essential to life. Those who strive against this spirit of change are striving against life, and thus are clearly sick and weak and wanting to escape from life.
The overman, however, is the full realization of a healthy will to power. He has gained complete power over himself, so that he is entirely a creation of his own will. His character, his values, his spirit are all exactly as he has willed them to be. In that sense, the overman is totally free and absolutely powerful.
The scholar, Deleuze, ties Nietzsche's notion of the eternal recurrence in with his notion of the will to power. The will to power suggests that the universe is in a constant state of change, so that there is no such thing as being; there is only a state of becoming. Deleuze cryptically remarks that returning is the being of becoming, and that the eternal recurrence thus expresses the fundamental nature of the universe. Only an overman can fully embrace the eternal recurrence, since only an overman can look at every moment in his life, and every thought or deed, as a creation of his own will.
Nietzsche followed up Zarathustra with ##Beyond Good and Evil## and ##On The Genealogy of Morals##, both of which were meant to give a more straightforward explanation of many of the main ideas in Zarathustra. If you have trouble with Zarathustra you might want to refer to one of these other two books, or to the SparkNotes written on them.