I recently finished the criminally underpublicised sci-fi FPS ‘Singularity’ (not a great game by any means but I personally really enjoyed it), before then playing the very much publicised sci-fi FPS ‘Crysis 2’. I’d be lying if I didn’t enjoy the latter but something that really struck me was the different in quality of the AI for each of the games.
I am not of the military ilk but the AI enemies in Singularity seemed pretty realistic, they would mill around and shoot the fat or follow set patrol paths when not alarmed but quickly move to engage when they spotted the player, they would quickly move to cover and pop out to let of a few rounds before hiding again, they would work in teams to cover the exits from a room and provide covering fire while other soldiers moved forward to attack, they would even on occassion fumble a reload in the middle of a firefight, which was a nice touch. In effect they were a difficult and believable opponent (at least until the point where the player characters weapons and powers completely overwhelmed then)
In comparison I was underwhelmed by the AI of the enemies in Crysis which was surprising given how much development and polish was thrown into the game, they appeared to do none of the actions above and even on occassion were comedic more than anything else. For example at one point my character was on the roof of a building watching private security forces on the ground below, so I power-kicked an automobile from the roof of the building so that it would land among the soldiers below…who didn’t react, in fact one character warned, ‘Keep your eyes open for anything unusual’!
Then there was the time I picked off an entire building of enemy soldiers who thought it would be a good idea to run out to the same spot as their fellows to check the disturbance, blissfully unaware of the increasing mound of bodies being stacked up in front of them. 
Anyway, any other examples of good or bad AI?
Edited to add I remember some time ago playing Soldiers of Fortune 2 (at least I think that was the title) which had the curious bug that if the enemy hadn’t detected your character you were unable to kill them, for example you could merrily lob grenades into a sentry post at a visible enemy who wouldn’t react until you walked into his line of vision, only then could you kill him. A bit suspension of disbelief killing and hard to imagine how it slipped past playtesting.