Loved the setting, not a fan of the fans.
I played exclusively in KoK though most of 3rd edition. It's biggest strength is the real-world-like setup. A lot of people say it's boring because there's not some huge big-bad sitting right around the corner. To me, that means I can do as I please and the players won't feel as though I'm hurting the core of the world.
Also, on the campaign book itself, don't read it front to back, read the areas you're running in and around and don't forget to read between the lines.
eg. Thybaj describes the buildings as short squat stone buildings, why? Because they mention constant giant raids in the past, for one (shorter buildings would be harder targets for throwing rocks) and two, there's subtle mention of earthquakes (which matches up perfectly with the tectonic plate map in the atlas, confirming a likely-hood of frequent earthquakes in the area. Shorter squat buildings would do less damage over all if they fell, where a multi level building would likely cause damage to surrounding buildings.
Those are the things to read into and you can find a lot of fun in the campaign world. It's also nicely setup for political games, since no one's really at "open war", but just about everyone has a case for it.
I can't reccomend the F&F series enough. The author, Wiggy, puts a lot of great ideas in his books and just from reading them you can come away with great character or plot ideas.
My one big criticism would be mechanics. The people behind the mechanics were not nearly as good as some companies and you had to really evaluate the classes, feats, and items. Most of them were severely under-powered compared to the core feats in 3rd ed, and the ones that weren't were a power-gamers dream (there's a feat that negates saves VS spells, one that lowers crit modifiers against you if you're hobgoblin), and the spellsinger was a good idea that turned out to be an empty class.
With that said, the game runs ABSOLUTELY great systemless. You can ignore every piece of crunch in the books (admittedly, there's almost none in their 3 coolest books, the CS, Atlas, and Goods & Gear) and run this game using the core 3.x material, 1e, 2e, or 4e, or hackmaster, if you're feelin' wacky!
If there were any books I'd recommend picking up it'd be: Secrets of the Alubelok Coast (a great region book that's full of fun plot-hooks) and Goods & Gear (An equipment book that, while full of pointless weapons, does give great information on clothing, food, coinage (including names and descriptions of every major kingdom's coins), and information on slavery and other illicit goods.
Happy gaming!
