According to the introduction in my book, Will to Power "contains passages of varying degrees of completion from Nietzsche's notes for a major work which he labored on for over half a decade and ultimately abandoned."
Book 4, on Discipline and Breeding, is the section of most interest and probably most recognized. Niezsche details the "Superman" and if you are thinking about reading Thus Spoke Zarathustra, the background provided will clarify Nieztsche's intent and meaning.
Nietzsche described his "5 Nos" that I summarize below, which I think are the main ideas of the work:
1. Feeling of sin; punishment
2. Christian ideals; Socialism
3. Weakened and moralized man; aristocrats
5. Scientific method, psychology; Hate against order and rank
I don't think this is his "ultimate" work. Will to Power was put together by Nietzsche's sister, so it brings up the question of how much Nietzsche really intended to publish. Everything does flow together, yet I felt it was long-winded. Some editing down would make it richer. I didn't think it read as anti-Semitic and for world domination through force as this book is said to be. Granted, his whole argument is about power. The only way to be happy, according to Nietzsche, is to have power. Without it, you can't be happy.
I was surprised by how much Nietzsche favored artists and art. His arguments against Christianity and Romanticism were solid, although at points he did seem whiny when it came to pointing out the flaws of others. His scientific arguments, especially against cause and effect, didn't really do it for me, and I think this is an indication of how old the writing is. Overall, it was a stimulating read.