Call it an RPG, call it an FPS...either way, its one of the top games of its genre.
The Story: Deus Ex takes place in a dark not-too-distant future that would have made Philip K. Dick proud. Overpopulation, pollution, and poverty are at an all-time hgih, the world his fallen into disrepair, rogue terrorist cells operate openly, and a mysterious plague is slowly spreading. You play a J.C. Denton, an anti-terrorist operative who has been implanted with nano-technology to improve his performance in combat. But there is more to everything that meets the eye, and as J.C. neutralizes terrorist threats, he unwittingly uncovers a multi-layered conspiracy. The constantly thickening plot is fabulous. Even if the gameplay sucked, the plot alone would make the game worth playing.
The gameplay is incredible. At first it looks like your average FPS, but it is much more than that. An RPG type stats system determine how effective you are at using certain skills, and since there is no way you can earn enough exp. to level up everything, you'll have to carefully choose what skills you want. The skills determine not only how you fight, but how you beat the various levels. For instance, lets say you need to get into an enemy building. If you have a high lockpicking skills, you can open one side door and get in that way. If you have a high electronic skills, you can hack the electric lock on another side door and get in that way. If you don't have enough of either, you can always rely on a mixture of guns and stealth to get you through the main entrance.
What really makes Deus Ex a great game is the monumentally huge number of options. You can pretty much do whatever you want in terms of whom you side with and whom you kill, and no matter decision you make, the game keeps on going, and your decision is worked into the storyline. Hell, there are three different endings to the game that depend on which faction you support.