It is a cross between AD&D and OD&D with some unique spins on mechanics and classes. Jeff T did a brilliant job on this.
It is a humanocentric setting that is Swords, Sorcery, Cthulhu, and some weird science.
I've owned the first edition forever, and own several of the adventures. I don't play the game as a whole (I ran some of the adventures back when I got them) as I find it a little too dense compared to other TSR D&D based games, but I've stolen plenty of ideas and some mechanics from it. Like DCC RPG, it's a great idea mine, even if you don't play it.
Aesthetically, I prefer the unified (amazing!) art of the 1st edition. I have the PDF of the 2nd edition but never did anything with it- it seemed "fluffed up" with presnetation and art mainly, but did add a fair amount of new stuff (classes, etc). The art is good in the 2E book but it the styles vary in significant ways, and IMO is jarring to look through. YMMV.
I don't think I'll go in for the 3E KS when it's live, just because I know it would be a shelf queen and see little use.