Typically 3-5 but I already have a lot of 3-5 player games that are fun for 30 minutes. Still, it's not uncommon for us to have six people and I've only got a couple of games for that situation and only one (7W) that isn't a 2+ hour game. So the six player thing would fill a niche and that's what got my attention. I mean, besides giant monsters stomping all over Tokyo.
The other thing is that most of the 6P games I have (Power Grid, Dominion, Arkham, Battlestar) are all games that really aren't great (or just flat out boring) with six, usually because they start to drag on or the difficulty changes or whatever. Cool as it looks, if KoT is one of those games that's really only good for 4-5 but technically it supports 6 I'll probably hold off for now.
I think one of the things that makes 7W and RoboRally fun even with a lot of people is that the turns are all simultaneous and there's less waiting for other people. Not really sure how KoT plays, though.