Zarathustra is not Nietzsche’s greatest work, but it is one of his most interesting. In a departure from his other works, Nietzsche abandons his aphoristic style (with some exceptions) and instead embraces an approach that consists of more narrative and poetry. This is both its great strength and its great weakness.
It is a great strength in that it allows Nietzsche to explore his unique ideas (especially the eternal recurrence, the Higher Man, and the death of God) with more depth than in nearly any of his other works. Nietzsche adopts a negative approach to philosophy in most of his other works, gleefully smashing his way through the idols of the marketplace. Here, in Zarathustra, he comes his closest to formulating a positive philosophy.
This philosophy is one that presents a challenge to any perceptive reader. The Christian, for example, will be challenged by Nietzsche’s razor-sharp criticisms of Christian beliefs; Nietzsche’s critique is not to be lightly dismissed. The atheist, on the other hand, if he is an attentive and sympathetic reader, will be challenged by Nietzsche’s own exuberant, even mystical, atheism, a far cry from the dry scientism that predominates in atheist circles today.
The style and content of Zarathustra also represent a weakness of the work in that one must be familiar with other works of Nietzsche in order to understand and appreciate it as well as one should. To this end, I particularly recommend reading Beyond Good and Evil, Human, All Too Human, and The Gay Science before one plunges into Thus Spoke Zarathustra. So properly prepared, a mindful reader of any particular philosophical persuasion will inevitably benefit from the monumental genius that was Friedrich Nietzsche.