Theres next to no games with good AI, normally its just one feature they've added, that is no different to any others, like flanking the enemy, it seems like an improvement, until you realise its one of two things, most games, duck behind nearest thing, in that game they just told them to move sideways more, its not really more complex, just slightly more effort put into thinking about what the AI should do, the AI itself is no better.
As for Stalker, the AI is abysmal in nearly all situations, there are very very few situations the AI "seems" inteligent, and these are just incredibly heavily scripted events.
The vast majority of "good" AI in game is just heavily scripted to appear like AI, while its actually dumber than normal. THe first stalker game, largely the AI is horrible, it does stupid things, its only in 2-3 situations it seems good after you leave the first area and end up in a junk yard, the AI is completely different to almost the whole rest of the game, they come in, in a big group and actively seek you out which they do not do anywhere else in the same way. However, its just a heavily heavily scripted event, a series of choices pre-determined, it can seem smart, but its clearly just scripted.
THe reason its not the same through the whole game, scripted events will only work in specific situations, in that one you basically have no choice but to do that mission, in more on the rails shooters, AI is even more stupid and scripted, in the rest of that game where the player has more choice, the AI has less. Frequently in Stalker if you stand a certain distance the enemy will switch between running at you and running to cover, sometimes it will just run from one end of a long boulder, or bus, to the other, and never actually do anything else.
AI is a misnomer, the vast majority of "good" game AI is down to the time and number of programmers they dedicate to creating scripted events.
As we get a bit more power, it gets easier to add another choice to AI, but its not any more or less complicated.
I've been completely unimpressed with the increase in AI over the past, well, as long as I can remember, shoot + duck is the vast majority of what you encounter in most games, shoot + duck + move a little is almost as complex as it gets. Shoot + duck + move when another friendly is shooting is just about the most complex thing I've seen in games so far.