STALKER sets a high standard that too few seem to appreciate.
The military guys are usually more powerful than you, so you have to out-play them. Head shots are your best friend. In fact, you cannot possibly hope to beat many parts of this game unless you make a strong effort to get as many headshots as possible. This makes sense considering most people are wearing thick-ass body armor. Bandits don't, but you generally have to fight several at one time.
The difficulty level of this game is why it is so fulfilling to play. If you are able to take out 10 military dudes in a row and survive, you feel like a badass because they are often equally as strong or stronger than you, and you retrospectively realize that you beat them simply because you are better at playing than they are. It doesn't get much better than that.
The shooting dynamic is about as realistic as I've ever seen. Your guns sometimes jam up so you have to reload, which is annoying, but so refreshing. Each gun (and there are LOTS of guns) has a different set of specs, IE, for accuracy, damage, control, etc, so certain guns are far superior than others in certain situations. In most games, an automatic is an automatic and there is little variety between them. Not the case with STALKER.
There are a few weak points of course. The arena, while awesome, isn't long enough, but there are some cool fights. In one of them, you have to take on two guys that are more powerful than you, but you only have a few grenades to fight them with while they have powerful automatics.
Another issue is the graphics. They seem a bit dated, but I personally don't really care considering they are optimized perfectly for the atmosphere that is being attempted. There are also some glitches I encountered, one of which was a quest for one of the factions: The quest giver somehow died but the quest continually placed a map marker to talk to him, rather than acknowledge that he died. This sucks because it was for the faction quests, which is apparently a huge part of the game that I never had the opportunity to do since the guy died (Skull was his name, if I remember correctly).
The real flaws with the game are somewhat nitpicky though. I've noticed that most people dislike STALKER because it is very hard and often frustrating. If it weren't for that, I'm sure the game would generally get higher ratings. It does talk a few hours to really begin to love the game... The first bit is confusing until you realize that it's supposed to be confusing. You did, after all, lose your memory and wind up in an odd STALKER camp with a bunch of strangers.
This game can really be appreciated by any FPS fan, but if you aren't hardcore into games and looking for new challenges, it will take awhile for you to fall in love with STALKER. Once you do, though, you'll go back to other FPS games and find them easy and generic by comparison. STALKER sets a high standard that too few seem to appreciate.