Ahead of It's Time Yet Still Unmatched
There is much that can be said about Deus Ex and it's quality as a video game, from the solid FPS gameplay to the interesting and complex story to its freedom to its RPG progression, but perhaps the best indicator of how truly a great a game is that even amongst more recent FPS/RPG hybrid games you can't find anything as great. Games like Bioshock, even though that game makes it far on atmosphere, feel watered down in every field that Deus Ex excelled. The only other game that can stand alongside it is System Shock 2. Which you feel is better is more of your preference: conspiracy or horror.
In the end it's a must play and the most fulfilling game I've ever played. It's playable over and over, even at a 40 hour length, and that's the most convincing thing I can say about it.
