Assuming you haven't played it, the series is basically the conspiracy simulator the same way Fallout was a post-apocalypse simulator. It's got you fighting directly with the Illuminati, Majestic 12, government conspiracies and Area 51 alien stuff. Basically, anything conspiracy theorists can think of, you can find in the Deus Ex series.
Deus Ex: Invisible War was the Fallout 3 of the Deus Ex series. It has its good parts, and is somewhat a little below average as its own game, but is completely trash as a Deus Ex game by throwing away depth and choices in favour of streamlined console FPS style garbage. But it's still appreciated by many people who don't really understand why it gets so much hate.
Deus Ex: Human Revolution was the New Vegas. It has a touch of modern linearity and streamlined console influences to draw it back from being as much of a gem the original was, and due to AAA graphics, it was incapable of reaching the sheer scale of the original, but it captured the spirit of the gameplay and writing in the original. It was the proper sequel and everyone refuses to acknowledge Invisible War as being part of the series.
Deus Ex: Mankind Divided, the upcoming one, is looking basically like what happens if New Vegas 2 was made instead of Fallout 4. With the same Courier and all.