S.T.A.L.K.E.R. is an RPG best suited for shooter junkies, not the best shooter for RPG fans.
The game's shooting mechanics are solid. There are numerous weapons to pick up, and all of them behave realistically. You will not be very accurate using your weapons from a distance, so the game really forces you to get in the face of you enemies. All the human NPCs behave really well. They'll find cover when fired upon, and will work together in an attempts to eliminate you. The A.I. in S.T.A.L.K.E.R. is by far some of the best I have seen in games. The game works really well at creating a distinct and immersive atmosphere. The graphics in the game are nothing amazing, but the combination of many visual effects and killer audio all work really well together to really pull you into the game. While the game functions really well as a shooter, the RPG elements leave much to be desired. The ability to talk with every friendly character in the game does little to help the player progress in the game. Picking up side quests offers little reward for the player, as the most powerful weapons and armor can be obtained from playing through the main quest. Picking up side quests only offer more money and ammo. The games offers even less incentive to explore most of the world. There is very little to be discovered, and the equipment hidden throughout has little value.
The shooting mechanics and atmosphere will delight any fan of a good shooter, and hours of enjoyments. However, the weak presentation of roll playing elements will leave RPG fans wanting more out of this game.