To stop the New World Order or to join it? This rests in the hands of just one man.
It's perhaps the only game that has treated conspiracy theories the better, using stuff from real theories and changing the name of some other more polemical things (like "The Grey Death" disease emulating a well-known disease of our times).
The upgrading and personalization of your character (like in an RPG) is one of the main keys why this game is so good: you can become whatever you want, a warfare machine or more stealthy than that guy from that Tom Clancy game
The game also counts with lots of choices to be taken. Your actions reflect on how the game evolves. For example, important characters to the plot can die or survive depending on your actions (saving them from a planted bomb or helping them when the enemies overcome them, those are two of these situations in the game), you can also face problems in different ways, by talking or being unnoticed or by opening your way with fire power.
The character and plot development is also very good. In the beginning (and if you aren't aware of conspiracy theories) you would never expect one of the biggest plot turns in the game, so you never know who you can trust in the Deus Ex world (like in the real one, let's face it...).
The music is also one of it's strong points, the themes can be epic, relaxing or thrilling.
I missed some things from the port to PS2, some of the music was changed and some levels are shortened, but well, can't complain, could have been a lot worse.
Also, some levels felt a bit empty and quickly built. Sometimes it results in a boring exploration experience for the player (maybe it's because I don't like shooters very much).
The Bottom Line
If you like conspiracy theories or a good RPG/Shooter with character and plot development, Deus Ex will be a wise choice.