I got this game for Christmas... a little late on the uptake but I only just got off my lazy backside and got a decent PC (needless to say, my 5 year old laptop was lacking).
I'm not generally a fan of shooters, but I'd heard this was a really indepth game. When I first played it my instant reactions were "Wow!". Then I swiftly ended up full of lead... The game is very fun but very difficult, it really does follow realistic rules as a shotgun at close range, or a few well aimed rifle rounds is all it takes to kill you, even when heavily armored. The rule of thumb is stealth, if you can be stealthy about your kills you'll be fine, whether that be a well aimed nade, or sniping enemies from behind a bunch of cars with a silenced rifle, its the easiest way to win.
The graphics are really top end, but unfortantly my PC didn't like the X-Ray engine and refused to play with dynamic lighting, which adds to the atmosphere and detail, but all in all it only crashed for me once and that was a corrupt save file. I've seen a few bugs, such as guns being hidden under the ground, players walking through walls, or optional missions not being completeable due to bad spawns. The only bug that has really annoyed me is in a town, I got a bad spawn and a bunch of enemies entered the map, I killed one and the town killed me, beyond that the other bugs are easily ignored.
The atmosphere is fantastic, on a number of occasions I've had my heart racing or pretty much jumped out of my chair, playing in the dead of night makes the game even more enjoyable ;)
Overall, very good game, the optional misions are a nice addition to draw you from the main missions if you want, all 3 endings are dissapointing in my opinion, but the rest of the story is great, and the 3rd (and by far best) ending is extremely difficult, makes the rest of the game look like childs play.