I've lost track of how many times I've nearly pissed myself because an enemy NPC crept up behind me and put a round in the back of my head while I was busy doing something else. I'm not sure which of the STALKER games has the best AI but it can get really clever. I've never played another game that has enemies that can spot you from a distance, recognise that you are an enemy and that you haven't spotted them back, and then sneak up to you to assassinate you stealthily. You might've never even known they were there in the level with you until you're staring at the game over screen.