Great game, great substance, old technology. Still a must have.
Let me first say that this game has its high points and low points, both of which are very apparent when you start to play the game. I will start with the positive aspects of the game.
The best thing about this game, by far, is its AI and open ended nature. The open endedness of this game really sets it apart from other first person shooters. While it is not as seamless as other open ended games, such as Oblivion, it still is extremely open ended for an fps game.
The environments are restricted by barbed wire fences and are fairly limited, you can't keep walking in a direction for more than a minute without hitting a border, and you pass between areas through small connecting gates (think fable). However, this does not detract too much from the game, and as I previously mentioned, this is a HUGE step up from other fps games.
The AI in this game, simply put, is some of the best I have ever seen. The enemies in battle are very intelligent, take cover, act as teams, and flank in a realistic way. Enemies back off and flee when they are outpowered, and their alertness isn't omniscient as it is in many games. What is even more amazing than the human AI is the creature AI. The creatures act like unpredictable creatures should. In this game, I approach animals as if I would a stray dog in real life. They protect their food, shy away when you fire a rifle (usually) and attack if threatened. The world around you is full of consistently occurring AI encounters. Once, while walking along a road, I observed some mutant dogs hunting these boar creatures for a meal.
On a final note, the actual combat in this game plays out nicely, despite the annoyance of enemies taking 10 shots to down if they aren't hit in the head. Seeking cover and using tactics is necessary, running and gunning (while possible) is usually not the best approach here. The bullets travel, penetrate materials, and ricochet realistically.
The negatives in this game are not as significant as the game's strong points, but are still significant enough to be noted.
The first point is the sound. While not all terrible, many of the guns sound weak and really don't make encounters as exciting as they could be. The sound is not all bad by any means; the game makes full use of EAX, and the environmental sounds are great. The wind whispering in the trees, distant cracks of rifles, and scary creature howls all fit in nicely.
The games graphics, much like the sound, are a mixed bag. The environments, usually, are fairly nice. The areas are covered in vegetation and have nice textures. Trees and grass blow in the wind, and the shadows and lighting are really nice. However, the models and weapon models are pretty bad. I make special emphasis on the weapon models. Sometimes in this game I walk into a barren area and am slightly repulsed by the graphics and low polygon counts, and other times I am amazed at the amount of ambient lights and shadows cast from my flashlight as I proceed through a wrecked hangar. Overall, the graphics are more passable for the last generation of games, but the gameplay more than compensates for this. The plot in this game is pretty weak, there are no characters to speak of (except for yourself and random stalkers ) and the story is almost exclusively told through text boxes. The map/quest interface is slightly irritating, but far from unusable.
As a final note, do not hesitate to buy this game. It isn't a straight up shooter, and provides a sense of immersion and tension I haven't encountered in an fps since I played System Shock 2. The game is a unique blend of first person action, role playing elements, and a strong survival horror flavor.