As far as "bang for your buck" goes, Planes of Power is in my top 5. Fun gazeteer seeds one can easily build a campaign around for every plane, plus new monsters and a political primer on the genies. If you want to take your party into the elemental planes, this book is worth is every copper.
This is like candy for those of us that use a lot of fey in their campaigns. The Eldest getting full demigod workups, actual rules for First World adventuring (and having had to improvise rules for this before, I wish I'd had this book a year ago), and especially the dozens of location seeds for the First World all contribute to make this one of my favorite setting supplements. Also it does an amazing job incorporating the farther-flung reaches of the Golarion lore in ways you wouldn't expect (no spoilers here).
My only criticism lies in the "Feysworn" prestige class. Hilariously broken (1st level makes you immortal) and clumsily edited (no entry for class hit dice, and some class abilities are presented out of order). This is a great prestige class to apply to an NPC mortal servant of an Eldest (and I've already done so), which is probably the idea from the outset.
Besides the prestige class, this book is in my top 5 Chronicles purchases. Well worth the money if you want a fey-heavy game.
