Deus Ex: a classic of video games.
With the thousands of player reviews, I can't add much beyond saying Deus Ex presents an engaging storyline with philosophic debates. You are quickly immersed into the game world and, by the choices you must make, feels more alive than many recent RPGs. The world depends on you, not to determine it's destruction, but it's control. Where one choice might grant freedom, it will also plunge the world in chaos. If you want to create peace, you must influence and control every aspects of humankind's lives. Throughout the entire game, you find yourself thinking about the repercussions of your choices and if you did the right thing.
As far as gameplay goes, Deus Ex presents great stealth mechanics (keeping in mind when it was developed), good shooting and a large array of skills and abilities (biomods) to train and upgrade, to allow all manner of approach to any given situation. You can be a juggernaut of destruction, specializing in heavy weapons, upgrading the Aggressive Defense System, Ballistic Protection and Regeneration biomods , or take a more stealthy approach, jumping from incredible heights and taking your sniper rifle to kill targets from a mile away.
All in all, you musn't miss on Deus Ex, and now is as good a time as any to pick it up -- it's 5$ and Human Revolution will be at our door step soon.