System Shock 2 is a game that is great fun! There are many aspects that helps this game stand head and shoulders above most PC and console games. The story is that you are a crew member on a large ship that is exploring the galaxy when something goes very wrong and as a result zombies with worms on their necks start ending peoples' lives. And then the fun begins! Firstly, SS2 is genuinely difficult! The enemies are hard and you need to think twice if not thrice before you fire more than a single round at an enemy due to ammo's scarcity; you need to make really difficult decisions, such as whether to upgrade your hacking skills or your research skills etc.
Secondly the atmosphere is what really sells the game hands down! I think I got into the game when I walked into a common area littered with dead bodies with a message of "Remember citadel" written with blood on the wall; SS2's enemies are creepy too as they walk aimlessly through the halls moaning stuff that freeks you out when walking through a remote corridor.
There are also audio pads that are left behind from the now mostly deceased crew that helps you find out what happened to the ship that allowed this disaster to happen and what you can do!
In conclusion, SS2 rocks!