Matewis: Stalker has this incredibly annoying bug related to stealth. The stealth for me usually worked ok, but only on the first try. Once you alert enemies and load a save from a little earlier to try a stealth approach again, you will find that the enemies are still alert.
Yeah that's a separate thing but for stealth to be what I consider not broken, you should be able to sneak and hide even if the enemies are alert. This doesn't seem to work at all. If they're alert, they will see you in pitch black darkness and through bushes and foliage, even if you're low crouching and not making any noise. What happens to me a lot is I'm just reacting to muzzle flashes in the bushes and taking down guys that I never see. Likewise in the underground establishments.. the first one you enter, it's pretty much guaranteed that there's going to be a gunfight. What happened on my first attempt is that I was crouching in a pitch black room, I couldn't even see the sights of my gun and someone just walked into the room and shot me (without a light or anything). In games where I consider stealth to be working, you would've been hidden in the dark and the enemies would've been just looking around (alert but not aware of your location).
Speaking of noise, I also find that the enemies are very sensitive to it in some places. Like the very first place where you're likely to engage bandits in the game. Some guys hang out inside a building. You can walk by at night and they will hear you through the walls and rain!
idbeholdME: As for Stalker, I highly recommend the Zone Reclamation Project. It fixes a huge number of bugs and glitches.
There is also Sky Reclamation Project for Clear Sky.
I'm doing Autumn Aurora. I found the vanilla gunplay to suck ass and it didn't seem like ZRP does anything to fix that. AA changes it completely to something I mostly enjoy (except for that lack of proper stealth).