can anyone point me towards a game where the devs actually spent a significant amount of time (relatively speaking) on the AI?
Chessmaster 9000?
For most games, though, AI is an unlimited time sink; every thousand hours you sink into making the AI smarter will barely enhance the player's enjoyment.
Or, often enough, will make the player's experience worse; I'm pretty sure there's no way I could possibly single-handedly kill hundreds of bad guys unless those bad guys were complete idiots who run towards me one at a time and never bother to hide, strategize, cooperate or even pick up the powerful weapons which are just sitting around on the freaking ground.