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 ... This is where D&D became AD&D, honestly.