I read some very good reviews on this site of KoK and resolved to get hold of some of the material. This was about the time that Villain Design Handbook got published, and I'm a sucker for "GM advice" books, so in fact I bought that before any setting stuff.
VDH is one of the worst 3E books I've bought. Let me qualify that ... it has the usual prestige class, spells, monsters, etc. padding, but if you are looking for the stuff it says it does on the back of the book, around evil character design, then that material is very weak indeed.
OK, I thought, wrong book. I bought the KoK core book (more money than sense). Now, this book turns out to be very dense with information, but as a setting I frankly found it uninspiring. You can (as I've heard others say) mine it for stuff, and it very likely ports well to your own campaign, basically because the overall setting is fairly shallow when it comes to context. Someone then told me that I really needed the Kalamar players handbook (or whatever it is) AND the core book to understand the setting. At this point I made my excuses and left.
I own core books from the following settings - Forgotten Realms, Scarred Lands, Arcana Unearthed, Iron Kingdoms (I'm counting Monsternomicon/Lock&Load as core), Harn (and, stepping outside D20, ShadowWorld, Runequest, Tekumel). I'd say all of them are more interesting settings than KoK. Those people wanting to compare realism between Harn and KoK - sorry but Harn wins hands down.
For those people who say FR is "unrealistic" in fantasy terms, well yes, but I do think FR has a strong identity that comes over well in the core setting book. People who buy that book have a certain expectation as to what they are going to get, and I don't believe they would come away unsatisfied.
I do find myself occasionally pulling KoK entries for my own custom campaigns, but I wouldn't miss it if it wasn't there. I'd grant some people seem to really like it. My general take was that I wouldn't recommend it.