Full disclosure, I was a backer for the Kickstarter for the Forest Kingdom Compendium and even had two fey queen sisters added as rulers to one of the kingdoms. More, as a GM and Player I look more towards substance of lore, world building, and rules which allow me to make interesting characters. Measuring game mechanics and crunch is not my strength.
Still, I will try to be as objective as possible. Which means starting out, I have to say I love what has been offered and may even making a campaign with this using Wardens of the Wild as well as the other main resource.
This proved just about everything one might need in creating a more wilderness campaign anywhere from Arthurian Legend, to exploration and the pioneer frontier, building your own kingdom despite the dangers of the unforgiving land. Where the forest is felt to hold the greatest mystery and challenge.
From Explorer who maps out his travels, to the Fey Mesmerist that focuses on different branches of magic such as enchantment, illusion, nature, to light and shadow. There is the Greenweaver Kineticist, to the Hidden Guardian a mix of paladin ideals with the ranger focus. The Huntsman who calls on older spirits then many mediums, and the more satire or humor loving Jester Bard, to the healing focused Knight-Surgeon. or the Cavalier who choses to be the protector of the forest by joining the Order of the Woodlands.
Finally, one of my favorites, the Unseelie Ovate who is a more casting focused Druid who forgoes a domain or animal companion for magic of the admittedly darker fey as well to transform into a fey themselves. Throw in the Fey Spell Lore from Ultimate Intrigue and you can add even more spells.
You have different kingdoms to use, each with flavorful characters and interesting settings, no magic and magic items, rules and ideas for adding the fey into your campaigns, to rules and resources needed involving Royal Tournaments & Fairs.
The new spells may need to be taken with consideration, as magic is often the most concerning or divisive aspect of many games when it comes to balance, yet I haven't so far seen anything which stands out as being an issue personally.
I'd like to make special mention of feats section, especially the Shade of the Woodland line for evils Druids and villains for a more nature focused campaign. A line of feats which in Pathfinder lore make a character a servant of the dark and twisted god Zon-Kuthon. These feats, the way they interact with each other and help form a character is fascinating in and of itself. I have spent hours trying different ideas or considering how they could come together in created a character or encounter.
Great job on these and the other feats, perhaps leaving a player or GM wondering how they can get together the feats they want and what they have to take out. Just remember, Fairy Blessing or Shade of the Woodlandare feats which are a prequequisite to most of the rest and in some cases both.
All in all, I feel the Forest Kingdom Compendium is a great resource and in my opinion well worth the cost. It may not be for everyone, but for others it could be a well loved resource.