Great atmosphere, somewhat open-ended gameplay, and a little more intelligence than the usual FPS. Some problems though.
The breakdown:
Graphics - Pretty good actually. The overall look of the game is gritty and dark, which helps convey the not surprisingly dark and gritty theme. There isn't a whole lot of shiny or flashy, so this may lead some to the conclusion that the graphics are crap. The gameworld has lots of detail and it's clear that the level designers worked closely on every piece of the map. There are no copy-and-paste moments and each area looks unique enough to be memorable (as unique as a bombed out disaster area can get). The dynamic lighting is excellent and really adds to the mood, provided you have a good enough video card to pull it off. Lightning storms at night and the flashlight show this off nicely. There's also a day/night cycle, which certainly adds to the atmosphere, but can also be a nuisance as night fighting puts you at a distinct disadvantage against enemies who have no handicap on their sight or accuracy in the darkness.
The downside of all of this is that most people will not be able to play the game decently with the highest settings like dynamic lighting, max draw distance, max textures, etc., so the game may not look so hot after you've dropped far enough. I tried lowering the settings to static lighting and medium textures just to see what it would look like, and I must say it looked quite bad. I played the game pretty much with everything set to max, so my impression of the graphics quality should be taken with that in mind. The AA is next to worthless and does not provide nearly enough of a visual improvement for the performance hit incurred. Leave it off.
Semi-free-roaming gameworld - It doesn't have the openness of Oblivion or Far Cry, but it's not as railroaded as HL2 or Doom either, somewhere in-between. There are load zones between each map and boundaries limiting how far you can range, usually a fence (the utterly impassable barbed-wire fence) or a high radiation zone which will kill you. The maps are adequately sized, giving some room for exploration off the beaten path, though it doesn't take long to hit the boundary. You can go back and forth between maps at any point until the very end of the game, so in that sense it is much more open than the standard FPS.
The side quests for the most part are extremely generic and Fed-Ex. Get an artifact, mutant body part, kill this guy, kill off a herd of mutants, etc. The rewards usually involve small amounts of money, making the quests not worth bothering with most of the time. There are a few exceptions where you'll actually be given a useful artifact. Many of the quests have 24 hour time limits and end up expiring before you can get from point A to B and back to A again, having to traverse 4-5 maps in the process. Overall the quest system doesn't work too well, except for the main story which thankfully gives you an infinite amount of time to finish.
The combat can be challenging in the beginning as the early weapons have poor accuracy and good body armor is not readily available (though it can be found very early on if you know where to look). Ammo conservation is very important as you may find yourself running out of ammo near the end of a mission. Enemies don't drop much ammo and there's very little of it just lying around, so it always pays to stock up on rounds at the merchant (carrying 300-400 rounds at all times is a good idea). The AI is all over the place, sometimes smart, sometimes braindead. Human enemy AI will move around quite a bit and they can be difficult to pin down. The AI is not nearly as predictable as it is in most FPSs, lots of flanking and changing of positions. The mutant AI, however, is much more predictable, charging straight at you most of the time. When the AI works it's great, when it doesn't, well... There were a handful of occasions where the AI just stopped, their guns either pointed at the ceiling or ground, allowing me to run up and kill them one by one with no response. Mutants often cannot climb stairs and will keep running back and forth as the player picks them off from only a few feet away. These kinds of AI breakdowns didn't happen too often, but they do happen and it hurts the game a bit.
One of things that annoyed me the most about this game were the endless spawns. Characters (usually enemies - bandits or military) will spawn at map entry/exit points, so oftentimes you'll find yourself in the middle of a battle as soon as you enter a map. You can clear an entire map and only moments later go back and find that a whole slew of enemies have respawned. There needs to be some kind of cooldown period because as it stands it's out of control.
Scripted sequences can also get screwed if you save in the middle and then reload, the Duty/Freedom missions for example. The AI will forget what it was supposed to be doing and run straight into walls, etc.
One's feelings about the game will likely be impacted strongly depending on the type of ending one receives. The true endings explain the situation fairly well, tie things up for the most part, and one of the options opens up a lengthy battle sequence before the game is finally finished. The other endings are really a joke, and were intended to be a joke. They'll either make you laugh or smash the keyboard.
The strong point of this game is its atmosphere, especially the labs which do a great job at being creepy. The gameworld gives a decent illusion of being alive - stalkers sitting around campfires talking, playing guitars, packs of mutants attacking passers-by, etc. Immersive, an overused word, but it is that.
The weak point is a lack of polish. Quests can break. The AI can break. There are also some balance issues which need to be addressed - overspawning, deterioration of weapons/armor, jamming, money adjustments. My game was stable - only a few crashes to report, not unusual for most games these days.