More than any other Paizo AP (in fact, more than any other campaign I've run in 33 years of gaming), Kingmaker requires legwork for the GM and lots of it. Not only do you need to run the players through the adventure as presented, but if you want to get the most out of it you need to create scores, if not hundreds, of vibrant NPCs, give each PC their own plotlines to develop, and think several game-years (at least) into the future. The result can be an unparalleled immersive experience for everyone involved, but make no mistake, it takes a LOT of work to make it so. Therefore, anything that can make your life easier as a GM is something to buy, treasure, and recommend to others.
The first thing to take into consideration is the title. I think it's a bit unfortunate, since this is far from just a PLAYER'S reference; GMs will find much to love here.
I purchased the PDF version, so my comments are limited to that. Physically it's a nice book, with a lovely cover, good B&W illustrations throughout, clean text and presentation, and not a lot of page background to mess with readability or devour printer ink.
The first section deals with exploration and is essentially a restatement of the rules in Stolen Lands, with a couple of nice additions like a size-comparison chart showing how big a kingdom is with RL comparisons. It's fine, but nothing thrilling.
Next is city and kingdom improvements, which is where the book starts to shine. All Paizo's buildings are listed, but additions are made for cities (like Office of the City Guard and Witch's Hut), rural areas (like Apiary, Winery, and Royal Preserve), and even castles (ranging from a fancy art collection through a moat to an anti-scrying room). This is where the book starts to become indispensable, as much time, effort, and balancing will be saved by having these structures ready to hand, in addition to giving players more of the options they crave. A much-improved random events table is also included.
Next up is mass combat, and the additions here are very useful indeed, ranging from new army types (everything from various size militias through orc raids to everyone's favorite shambling undead cannibals) through new attributes (like bleed, crusader, and mercenary) and rules to generate the sort of humanoid hordes we all know and love. If you plan to feature a lot of mass combat, you NEED this.
A short section on new Feats follows, which gives both traditional (e.g. Armored Swimmer or Tumble Strike) as well as Kingdom (e.g. Aid Another Leader or Inspiring Ruler)and Mass Combat (e.g. Mass Combat Focus and Inspiring General) options. Again, for someone really wanting to get into the building rules that make this AP special, this is excellent stuff that you will make use of.
The next section is for new spells, and it's here that the only real problem I have with the book comes, in the form of the various "Summon Army" and "Summon Nature's Army" spells. They're only usable by mass combat units, but I still think the present serious balance issues. Still, YMMV, and I'm sure some GMs will find them perfect for their games.
A very interesting section, somewhat misnamed as "Secret societies and organizations," comes next. it has two 5-level prestige classes and a pair of new archetypes, all of which are good, useful, and interesting, but nothing in it can replace, say, the sort of thing found in the Faction Guide if players wish to found their own organizations.
Two pages of magic items are the last major section, and these are generally useful, especially the magical statues that grant kingdom bonuses when placed in Parks. More could have been done with kingdom stat-modifying items, but that's a quibble.
Rounding out the book are several pages of revised forms for kingdom tracking, which is fine, although I think the majority of GMs find that the kingdom must be tracked through computer spreadsheets. These forms are fine and you'll like them if they're the sorts of things you like.
Overall, this is a truly exemplary resource for players and GMs using the Kingdom Building subsystems in their games. It's not perfect, but it is clearly a 5-star product and an absolute steal at its price.