There's so much I could say about this game, as it's easily my all-time favorite. To summarize, Deus Ex is one of the best cyber-punk, dystopian, conspiracy-driven adventures ever created in the video game medium. Sure, the graphics and A.I. (original Unreal engine) are dated, but storyline, atmosphere, and hybrid FPS/RPG gameplay easily make up for any shortcomings.
Plus, with HD graphics and gameplay mods coming out, it only gets better.