Lots of good reviews on this game already so no need for me to spill much ink on the vanilla game.
Shortly i consider this the weakest game in the series - yet it's still head over shoulders better than many other more known FPS games. It's gritty world is unique and i have always had fond memories of this game series (minus the endings for Clear Sky and Pripyat)... But no more on that.
So why do i play this? I play this because of TFW mod (The Faction Wars) - Few know about this awesome mod that gives you a ton of gameplay by removing the story and remaking the world to support factions. It's a free world now yet the area is so familar.
In this mod you start out as a loner without any equipment and it's up to you to try to survive. You'll need to eat, sleep and get paid. but that can be hard as there are no freebies in this game contrary to vanilla game. stashes are looted. corpses get looted right after the fighting stops and if you try to grab stuff that someone thinks belong to them they will confront you. Needless to say early game is hard but VERY rewarding. Later you will start doing faction missions - or continue to be a loner. It doesn't matter much what you choose. Point is the options are endless and there are so many routes to choose and before you know it the most limiting factor is your actual free time to play the game. I am looking forward to 50 hours+ of quality game time from this mod alone that executes just right the way faction wars should be.
Here is a good place to start looking at mods that might tickle you the right way: http://cheeki.violates.me/