I have all the stat-block books of the RPG line (except from Bestiary 6 =/ ). They are a must to me, as I don't want to expend my time creating foes again and again, and when I do create them, its good to have some examples.
There are dozens of NPCs in this book, with all sort of different classes (from hybrid classes to occult classes, or just the classic ones as well).While the classic NPC Codex arranged the many characters in it by "character class", this one arranges them by "Organization". The characters are all built to suit a sort of villainous theme, from evil cultists to secret society members to vile arcane scholars to evil druids. There are even pirates and thieves!
Villain codex has new gears, new spells, and some other options - the NPC Codex doesn't. These options to customize the villainous NPCs are also arranged by "Organization". You can also easily rearrange the members of the many Organizations to make new organizations, and the book gives some guidance on how to do this. Like this, you can use this book to insert many different foes in your campaigns "on the fly" (they are all ready-to-go NPCs, just like the NPC codex). Its ease to change their race, some class options, and the flavor, so you can adapt them to any setting or adventure.
Concluding:
The main point of the Villain Codex is, as with any "stat-block" book (Bestiary, NPC Codex, Monster Codex), to provide GMs with characters that are ready to be used on the table. There are some of options that can be used by players as well. You can somehow look at the characters in it to see how you would build your own Villainous NPCs, or maybe player can look at them and see how they would develop their characters class build.
Many are the ways you can use this book. I have it, and I use it all the time.
Unfortunately, I will be taking one star out of the five I wanted to give this book, as I believe we could get more high CR/level villains, to use as really powerful main antagonists in our campaigns. The most powerful ones in the book are CR 14... Powerful, but not so powerful.