...or horrible robots with chainsaws, which is a decent proxy for science.
So, this is a GM-focused book, but with a bit of PC material. The first three chapters are fairly quick, covering general construct construction, archetypes, and magic items. By and large, this section is decent but not exceptional. A few good archetypes (I particularly like the Construct Caller Summoner and the Voice of Brigh Bard), some interesting items (an Automaton Core is a minor artifact with actual methods for PCs or NPCs to get, if they're not very nice), and a bunch of golem manuals.
The meat of the book is the fourth and final chapter, which occupies 2/3rds of the page count. And this has some beauties.
First up are the Automatons, which are my new favorite thing in Pathfinder. Short version, in ancient days the Jistka Imperium was waning, and so as a way to preserve it some of the elders of the Imperium started uploading their citizens minds into thaumaturgical robots. Thus you have the living constructs known as Automatons, ranging from the adorable and familiar-grabbable Familiar Automaton to the mountain-shredding CR 20 Master Automaton. These guys have everything. Imposing stat-lines, gorgeous art, fascinating backstories, oodles of plot-hooks... you want it, you got it.
There are a bunch of new robots, golems, and clockworks. The stand-out here is the CR 17 Gladiator Robot, which is what happens when you give a hyper-effective killing machine a chainsaw and a directive to please the crowd. This thing is basically a slasher movie villain, and thus awesome. At the other side of the spectrum is the Clockwork Goblin, which is small and silly and kind of adorable.
Finally, there are templates, a half-dozen of them. Golems wreathed in caustic mist, constructs haunted by ghosts, constructs with artificial psychic minds, constructs with someone else's brain stuffed into them, commando constructs (as scary as they sound), hilarious malfunctioning recycled constructs, and the most awesome, steam-powered mechanical dragons.
Allow me to repeat: steam-powered mechanical dragons.
Conclusion: This is a GM book primarily, though a few of the archetypes are nice for a PC. It is a very good GM book, useful if you want any kind of constructs in your game.
Start with the Toxin Codexer, which pretty much single-handedly makes a poison-focused character actually viable. You can prepare poisons in your extract slots for free, their DCs scale, and you can pick up Discoveries that allow you to bypass species poison resistance (you do need to do know what you're facing ahead of time, so this is an investigator encouraged to investigate). That's the big draw of the book, but not the only stuff.
The Vishkanya, Grippli, and Nagaji stuff is all good, most notably the Dispelling Blood feat and Nagaji Spit Venom line. The Vishkanya Bard and Nagaji Fighters are also pretty interesting, and worthy of characters.
The new potion-friendly spells are useful and interesting. Toxic Blood is going to be a BBEG staple, and Glimpse the Hidden is a seriously useful spell.
The witch hexes are impressive, most importantly in their ability to give witches more Fort targeting effects.
As with any Companion, there's plenty of dross in here along with the gold (the magic items come to mind), but the gold is so shiny and excellent this a five-star book. One of the best recent Companions, up there with the First World booklet and Blood of the Coven.
Lots of stuff in this Companion, both mechanical (which is normal) and story (which is a bit more unusual). Of particular interest are the new psychic tricks, notably the Animus Mine (trouble with mind-readers and enchanters? No more) and the Psychic Marauder archetype, first psychic archetype since the class was released.
Of course, plenty of other things, including the Demonsworn Witch (have a demonic pact without being evil) and the Beastkin Barbarian (turn into a Triceratops!). Good stuff all around.