Could have been slightly better...
One of the more disappointing aspects of S.T.A.L.K.E.R. is the running involved. You character oftentimes must go on tedious missions that you spend most of your time running from one end of Chernobyl to the other, usually giving you disappointing and mostly useless rewards. While you're running from place to place, you can take time to admire the detailed and intricate environments around you. The weather changes from sunny to windy to pouring rain, and from night to day.
The sound in this game could have been tweaked much more. There are so many irritating and repetative sounds, mostly characters saying the same thing over and over. Something that can bring down the combat part of the game is the fact that the sounds do not give you any sense of where they are coming from. If there is something right around a corner in front of you, the sound you hear will make you think its behind you instead. The sounds themselves, however, are of the highest quality and are very original sounding. The music is spectacular, especially the main menu theme.
One thing that makes STALKER such a great game is that there are multiple endings and you can take different paths to get there. Sub-stories and side-missions will bring you different results in the end.
While it has several irritants, this is an excellent game and you should definitely give it a try. STALKER has a lot to offer and you will probably want to play again even after you've beaten it several times. It's a lot of fun and it deserves a lot of credit for its originality.