The worst thing about STALKER's AI is that, if you look at the code, theres plenty of stuff there, good stuff too! its just deactivated for whatever reason.
Here's just some stuff i found when poking around:
1) Stealth is absolutely possible, and easilly done! enemies have been told to allways hear you if you get within a range of several meters, turn that down to zero and you CAN sneak up on them and knife them, tweak their perception of sound a bit too and its actually quite nice!
2) More belivable AI can easilly be done by tweaking vision, at default, all enemies have a 180 degrees field of view (you only have 90), and can see quite far in all states, reduce their field of view and range a bit when in the "relaxed" state, and AI will nolonger spot you constantly, but can still see and put up a fight when in an alert state, this allows both stealth, and more realism.
The code even alows you to set the variables differently for people, so a military sniper has good vision, but a rookie bandit does not, again, this is not really used..
3) Enemies can be set to have a random chance of going into a panic when they spot trouble.. very funny, and belivable too! set it quite low, and most enemies will fight you, but every now and then one of them will wimp out and leg it!
4) Theres alot of random variables, at default thease are not really used, or used with so small margins it makes no difference, but if you tweak them a bit, you can have the AI react differently to certain things (like spotting you from afar, or not noticing you untill its too late), tweaking thease variables makes for much more unpredictable gameplay.
5) Night vision can be tweaked, so certain enemies dont see very well in the dark, or see's very well in the dark, this is barely used, all enemies see about the same.
And theres more, its all right there in the code, and if you have just a bit of modding knowhow you can easilly play with it (and enable stealth for instance!), seems to me all of this cool stuff that just sits there deactivated was very much supposed to be in the game, but since their dev-time was cut short, they didn't have the time to playtest it all, and decided to nerf it, so they knew the AI coulden't make too big a mess of things.
STALKER AI is not very good out of the box, but it can be! its just a matter of "some assembly required"
