Let me start by saying that Deus Ex is my third favorite video game, ever, behind Ocarina of Time and Mario 64.
I think this game is absolutely brilliant. The gameplay mechanics are flawless, the plot is intriguing and engaging, and the RPG elements are blended perfectly with the action elements.
Deus Ex is the story of JC Denton, who is a top agent for UNATCO, an anti-terrorism coalition created to counter the National Secessionist Forces. Over time, Denton comes to uncover a plot that extends as high up as the President of the US, and must fight to stop the conspirators.
The game, while essentially linear, goes about things in a very non-linear sort of way. You can choose to be a silent assassin, or a hacker, or whatever type of character you wish to be. You are able to upgrade different aspects of the Denton's abilities, and hone the ones you choose to use.
The game also features dozens of different weapons and items, all of which are unique and useful.
All in all, Deus Ex is one of the best games ever made.