While it is undoubtedly true that to read most recent critical theorists one wants acquaintance with the philosophical and anti-philosophical canons, Zizek is a different story. This is because he excels at giving coherent and surprisingly entertaining expositions of some of the most difficult thinkers in western thought (especially lacan, hegel, and kant). Reading Zizek will make you want to read these other writers, and Zizek's interpretations are as original as they are accurate, in both cases impeccably so.
The aim of the book is manifold. Among other things it:
1. Rehabilitates Lacan's thinking against charges of obscurantism (sokal, gallop, noel carrol, et al). This is particularly true of the chapters entitled "che vuoi?" (what do you want?) and "you only die twice." The former chapter is a tour du force reading of Lacan's infamous semiotic diagrams on the dialetic of desire (see last chapter of "Ecrits" (short edition)). Improbably, this reading is built up as a response to one of the most "mainstream" debates in all of analytic philosophy: Kripke vs. Searle, anti-deescriptivism v descriptivism. Ultimately the claim is that Lacan represents the Enlightenment ideals more than anyone else today.
2. It challenges the prevailing determinist interpretation of hegel and makes an exceptionally persuasive case that hegel is THE thinker of contingency and indeterminacy and "the opened", so to speak. This is the task the conclusion of the book takes up, starting with a close-reading of the difference between Kant and Hegel's thinking on the sublime. The thing above all in Hegel's legacy is the format of hegel's reasoning, the often misunderstood dialectical triad. Read the last chapter for a definition and application of this mode of logic.
3. Begins by making the case that it was Marx and not Freud who "invented the symptom." Another original and persuasive argument, not to be missed. In edition to a new notion of ideology and how to critique it, this chapter includes one of the best short introductions Freud's theory of dreams I know of.
People often deride Zizek as a "comedian/philosopher" who makes too much light of serious matters (see the new yorker profile of 2003 for such a take). And this book certainly has its share of dirty and/or political jokes. What this view forgets, however, is that while Zizek is perfectly capable of turning serious matters into jokes, it his ability to look awry at the most trivial matters, to take the big Other's jokes seriously, that is perhaps his most enduring quality as a thinker.