I both love and hate this book.
As the first "supplement" to the original D&D it did a lot of things to the game I don't love. It introduced complex multi-classing. (Though the fighter-mu elf was around in vol 1, we now have multiple species/class configurations and even three way splits like fighter/magic-user/cleric.) It brought in the half-elf. It introduced the thief and percentile rolls. It suggested the idea of chaotic player characters and parties prone to back-stabbing each other over treasure. It introduced variable weapon damage and monster damage. (Most people would say this is a good thing, but I've come to love the simplicity of d6 damage.) In general it just added more classes, more options, more spells, more ... and the rules complexity rises dramatically. This is where D&D became AD&D, honestly.