One of the best games of all time IMO, shame the genre is so underpopulated..

*** Be sure to get the mod pack that improves the graphics***

System Shock 2 is one of the few games in this genre (RPG/FPS/Adventure hybrid) sharing the space with games like Deus Ex. So if you've played Deus Ex you'll have an idea how the game "works".
What really makes System Shock 2 unique is the way it pulls everything off so well: The RPG element is well developed with a limited number of upgrade modules that can be spent in 4 different areas, including weapons proficiency /skill, physical attributes, Hacking abilities, and psychic abilities. There are numerous ways you can develop your character and get a quite different playing experience. Comparing this to Deus Ex it makes Deus Ex's RPG element look like a redundant bolt on. The adventure element is superb, its a well executed cyberpunk horror that keeps you guessing, updating the story along the way with the obligatory data pads and also communications from your mysterious helper. Each section of the game is something new so the story never gets old and you constantly have the feeling of being on the edge with unknown horrors ahead.

The FPS element is perhaps more rudimentary than it might of been in some ways but in others its also quite advanced, there is a large array of offensive weaponry and capabilities, the effectiveness of which are determined by your respective Skills/Cybernetic upgrades in that area. the firepower (or lack of) goes a long way to upping the suspense and atmosphere of the game, you're constantly low on ammo and having to use you're whole array of weapons A) because you simply run out of ammo on the gun you want to use B) because different guns and ammo types have vastly different efficacy on different targets. One thing that does feel a little artificial though and is a pain in the butt is that your guns are constantly degrading in condition and breaking, so you either need to be skilled in repair to fix/maintain them of just plain carry a load of guns of the same type so you can swap out when they brake - its unrealistic because in real life you might expect to fire thousands of rounds from a pistol like a glock without need for maintenance or it breaking where as in the game you might get a dozen shots.

What really makes the game, as i've said it the way these elements mesh together and support each other, they don't "feel" like separate elements but just part of the whole which is the game.