The original Deus Ex remains among my top 5 favourite games, so Human Revolution was going to have to be pretty special to impress, and somehow they've managed it. It stays very true to the formula that made Deus Ex so great and adds a load of new stuff which makes it even better. The cover system is excellent once you get the hang of it. The detail in the level design is impressive and the range of different ways you can approach each area is also impressive. You almost always have the choice to shoot your way in, crawl through a vent, pay your way in, find an entrance in the sewers, or even talk your way in using the excellent conversation system (there's even an augmentation to make yourself more persuasive).
One thing that is quite different from the original is that combat is a far more readily viable option. Most situations can be dealt with by shooting. Although you can still overcome every situation using stealth, it is just easier sometimes to go in guns blazing.
The hacking system does get a little tedious after a while. When you get the hang of it and it's actually simple - but a bit of a chore when you have a load of PC's to hack in one room. Of course this is the world of Deus Ex, so most passwords can be obtained by finding pocket secretaries or reading emails on other machines.
I had only one major issue with the game, and that was the boss fights. They are just dreadful, and are completely out of place in the game. Depending on which way you choose to approach the game, you can find yourself hopelessly un-prepared for the boss fights, for instance if you've focused all your augmentations on stealth rather than combat. Either way they are terrible - they go against the spirit of the game entirely, forcing you to fight to the death in an enlosed arena, which is in direct contradiction to the way the rest of the game is played. I'm almost tempted to drop a star from my rating just for this, but that seems a bit harsh as other than this miss-step it is almost perfection.
The game also drops an absolute ton of references to Deus Ex, and any fan of the original is bound to be intrigued and amused by many of them.
All in all it's a triumph, and you're going to get multiple play-throughs given the wealth of options available to you. I'm on my second play-through now, and I'm going for the ultimate stealth run. So far I haven't killed anyone. Next time I'll go through as a complete murderous psycho. It's worth it just to see the different reactions you get from the major characters and the way the story progresses depending on what you do.