Monster Codex is an excellent time-saving book for GMs, especially when they need to set up an encounter with D&D/PF’s most-common monsters: Orcs, Gnolls, Goblins, Bugbears, Hobgoblins, Ogres, etc. Fire and Frost Giants were unusual choices but I’ll take them. Never know if we’ll ever do a modified version of the infamous Against The Giants adventure.
Monster does utilize some classes outside the Core Rulebook so if you’re like me, those pre-made foes require work to modify; a Bugbear Anti-Paladin becomes either a Fighter or a Blackguard if it has enough levels to qualify. Keep in mind, Monster is providing leaders, sergeants, spell casters and unique NPCs you sprinkle into the rank-and-file versions (aka the standard/stock ones from Bestiary). The players discovering a Duergar Monk who can punch their lights out will give them pause.
The book is broken down by monster race in alphabetical order. Each section opens with new Feats, Archetypes and Spells these opponents may utilize. Then it closes with a new monster which is associated with them. They’re either a subtype (Flinds for Gnolls, Troggles for Ogres) or something they utilize for defense or livestock (Steam Boars in Fire Giant lairs, Yzobu found near Hobgoblin settlements).
Not sure if Paizo will be issuing any Codices in the cheaper, smaller, pocket form with Second Edition being a year away. They do sell it as a PDF for $10, then you only need to print out what you require as you go. Either way, Monster is a boon for GMs regardless of their experience levels.