Having replayed this recently i'd say this is still worth playing 6 years after release.

Visually it has aged pretty well because of it's focus on style and not realism.
Gameplay wise it feels just as good as it did years ago and doesn't feel dated.
Movement is quick and responsive, levels are expansive and with many ways to approach the objective.
Solid story with lots of story bits scattered throughout the levels in the form of NPC dialogues or books.

On top of that, it has HUGE replay value, usually i don't tend to replay games unless i've really really liked them, but this one just wants you to play it again. You simply can't experience the full game in one run, you most probably will not use all the gadgets at your disposal. On the next playthrough you can take a completely different route, try new gadgets/spells, go lethal/non-lethal (which will affect how the characters/world around you reacts to you), spare differrent characters to affect the story , not use powers at all etc...lots of possibilities.

Minor complaints:
I've felt like my full lethal playthrough was much more fun than the non-lethal, mostly because your non-lethal options are limited to choking the enemy and using sleep darts.
Face to face melee combat feels shallow.
Good ending forcing you to be very limited when it comes to how many characters you can kill.
Lack of new game+.

I would also based on my experience really recommend not to focus too much on beating the game without killing anyone on your first run, you are not familiar with the levels and will spend loads of time reloading the game which can become quite tedious, just go with the flow for your first run and do a pure no kill run on a subsequent playthrough.

For fans of Deus Ex, this is a must play, yes the world setting is the completely different, but gameplay wise these two are extremely similar.
Would also recommend it to players of games like Thief, Hitman and to some extent Bioshock.

If you prefer being guided through the levels or/and don't like being overwhelmed with options and just want a linear experience, this probably isn't for you.
For everyone else it's a must try.

Having replayed this recently i'd say this is still worth playing 6 years after release. Visually it has aged pretty well because of it's focus on style and not realism. Gameplay wise it feels just as good as it did years ago and doesn't feel dated. Movement is quick and responsive, levels are expansive and with many ways to approach the objective. Solid story with lots of story bits scattered throughout the levels in the form of NPC dialogues or books. On top of that, it has HUGE replay value, usually i don't tend to replay games unless i've really really liked them, but this one just wants you to play it again. You simply can't experience the full game in one run, you most probably will not use all the gadgets at your disposal. On the next playthrough you can take a completely different route, try new gadgets/spells, go lethal/non-lethal (which will affect how the characters/world around you reacts to you), spare differrent characters to affect the story , not use powers at all etc...lots of possibilities. Minor complaints: I've felt like my full lethal playthrough was much more fun than the non-lethal, mostly because your non-lethal options are limited to choking the enemy and using sleep darts. Face to face melee combat feels shallow. Good ending forcing you to be very limited when it comes to how many characters you can kill. Lack of new game+. I would also based on my experience really recommend not to focus too much on beating the game without killing anyone on your first run, you are not familiar with the levels and will spend loads of time reloading the game which can become quite tedious, just go with the flow for your first run and do a pure no kill run on a subsequent playthrough. For fans of Deus Ex, this is a must play, yes the world setting is the completely different, but gameplay wise these two are extremely similar. Would also recommend it to players of games like Thief, Hitman and to some extent Bioshock. If you prefer being guided through the levels or/and don't like being overwhelmed with options and just want a linear experience, this probably isn't for you. For everyone else it's a must try.