The Forest Kingdom Compendium is a compilation and expansion of the various supplements Legendary Games have released which support (in typical Third Party fashion) the Kingmaker Adventure Path. At this point I will immediately say - if you don't have, don't want, and aren't going to play that AP, that should NOT prevent you getting this book. It's generic enough (within the areas it focuses) to be useful.
There's Everything. Okay, that might be a tiny exageration, but not much of one. It's called The Forest Kingdom Campaign Compendium. And the only one of those words that's even a little bit extraneous is "The". Archetypes, feats, prestige classes, spells, magic items, fey, royal tournaments, countries, characters, heroes, monsters and two adventures to supplement the AP. All of them with a foresty or kingdomy slant.
Subsystems FTW. I'm a fan of subsytems. I've been writing my own since the 2nd Edition D&D days (most of them not very good!), and I love reading new ones, and this book has a few (mostly involving fey and since I've got a possible nymph/paladin relationship of some kind in my kingdom campaign, those are awesome). The single biggest is Royal Tournaments (available as its own book), which is just crammed with amazing rules for running a festival, taking part in events at a festival, or just plain hanging out at a festival.
Beautiful art. I have a distinct fondness for fantasy art. Generally speaking, the more realistic the better. The art in this book is stunning. Yes, lots of it is repeated from other books (no real surprise), but just going to another page and seeing yet another pretty picture that perfectly fits with nearby text.
When is a feat not a feat? Okay, this is really nitpicky, but there are two feats in the section that describes faerie bargains with mortals, and they fit there thematically, but 90-something pages earlier are a whole slew of new feats for characters, and it's a bit weird for the feats to be separated.
Prestige classes. I love the idea behind prestige classes. I think the implementation of them in the 3.x/Pathfinder rules sucks. They were overused in 3.x D&D, and Pathfinder has done a lot to make them less of a feature of the system, and I think that overall the game is better for not having many. So the addition of three prestige classes rubs me the wrong way. That's not to say that there's anything actually wrong with the classes themselves (though I do have to question a 6-level class), just that I think I'd have preferred archetypes to fit the niches.
I love this book. Putting aside my personal dislike of prestige classes and questions of where feats should be in the book, it's an excellent resource for players and GMs, whether you're playing Kingmaker or not, if you're going to spend any amount of time in forests, meeting fey, or at kingdom fairs, then this book has something to offer you.
The things that I think are wrong with this book do not even put a dent in the amazingness that is everything else. I'd be particularly mean to deduct a star for those things, and this book is being integrated into my home campaign (which has nothing to do with Kingmaker except Kingdom building). Got to give it 5/5.