Well worth the wait.
Story:
The story of Deus Ex: HR sets up the beginning of the original Deus Ex. It starts out with a bang and then keeps you wanting answers until the later half of the game. Then when you get the answers you still want more! It is rare that you find a game where the main story keeps you so engaged that you find yourself fighting to actually do the side quests. Deus Ex: HR will do that. I thought the story was that good. There are multiple endings which I felt were rushed. However, sit through the credits and you'll be rewarded.
Mechanics:
The mechanics of the game are sound. The cover system is well done and easy to engage. The augmentations are really cool, and the stealth kill system is very well done. The first-person shooting is good, but its not BF or MW standards. There are also enough augmentations that I don't think you can unlock all in one play-through so if you want to try others you'd have play again.
That being said, the AI is pretty dumb and the only thing it has going for it is that if you rush in you take a lot of damage. They also only travel on predetermined paths so you can easily watch for a second then stealth kill them all. Also, I thought the inventory system could have been thought out a bit more (still not sure why you wouldn't let grenades stack but oh well).
Overall:
The games is a buy. I also think it is a keep. I will be playing through it again. It just has everything you. The graphics are amazing. The story is great. The combat and stealth system are so good you can play the game to your style. It is an amazing game. I think more companies should take more time to develop games like this, rather than pushing out yearly updates that aren't that good. Deus Ex:HR exceeds its hype.