Difference and repetition struck me as nothing I've ever read before has struck me. The fun thing about "reading" it, is that, when you think about it, the act of reading itself makes understanding parts of this work more clear. Reading this becomes a "machinic" activity as it were: immediate, affective, with its own unpredictability, with many gaps, moments of insight, despair, and so on. It seems contradictory, because I think it is the most rigorous and analytic of all of Deleuzes works. But it is immensely dense, as other reviewers also say.
It is certainly the crucial work in his oeuvre. Really if you have tried it a few times, you will notice that many ideas of his later work are based on the crucial notions of this grand exploration. Anti-Oedipe is such a delight to read and easy to understand after this one.

And I think it is good for those who want to approach Deleuze's thought, to start with the Anti-Oedipus and Mille Plateaux, then read some of the smaller and intensive works (What is philosophy, Leibniz et le Baroque). Then try this book. You will get many references and want to read all others once again.

It is clearly in this work that you will find the first monstrous and frontal attack against Hegel's dialectic. The fun thing is that this is a complete "anti-work". Every conceivable concept of modern philosophy (from the concept of "common sense", "history", or "being") gets an "anti", with which Deleuze consistently builds his grand idea of the immediate, the pre- or non-representational and the virtual--against any metaphysics. It is moreover his first, and I think also his last work where he builds his philosophy in a consistent manner.
After this one, I think he started exploring fragments of his thought more deeply, in his other works, which are derivatives so to speak. This is his goodbye to classic French philosphy (strong tradition of exploring the "history of philosophy") and his entrée into his own experimentation with the concepts he just developed.
To conclude, just some practical notes. The problem with the book is that, unlike his other works, you have to read all of it (because it is so consistent). This makes it a project for months, or even years. Good luck.