Man, it is fun as shit. This coming from someone who rarely finishes games. I still have Mass Effect, Mass Effect 2, The Witcher 2, Dragon Age: Origins and Dragon Age 2 that have yet to be completed. I usually get bored or, more often, don't have time. With Human Revolution I'm making time.
I wasn't expecting much to be honest. I felt, and still feel, like the golden age of (PC) gaming was in the late 90's/early 2000's when Nvidia and ATI were on the rise and 3Dfx was king.
When it was released, I played the first Deus Ex at a friend's house because I was a broke, lower-middle-class asshole who couldn't afford shit and smarmed my way over to their house just to see them play it. When I got a chance to jam, I was totally taken by the game. I was just like, "Holy shit, this is what a game should be."
Now with games getting more press and extremely hyped, only to fall far short of nostalgic expectations (mostly due to the fact that many games are marketed towards consoles whose expectations haven't been perverted by the quality we had back in the day on PC), I was a bit cautious about getting excited for DE:HR. I'd forgotten that I'd bought my copy in advance months ago (shows how 'in' I am with the gaming world) and only realized the day before release after a friend mentioned it.
I went in on the easiest mode simply because I like to get more involved with the story and less involved with running around searching for ammo. It's still difficult. You can't survive encounters with tons of dudes unless you've got the controls down. You need to hide and sprint everywhere when they've got a bead on you.
The stealth is excellent, as it should be coming from the remnants of the Thief franchise. It really mimics the intensity of Thief as Yahtzee described in his video on the series - when some dude is 'Altered' and comes waltzing around where you're hiding, breaking up his patterned patrol path in the process so you don't know where he's gonna look. Sniper patrols and cameras also make this interesting. Robots/mechs make it 'Terminator'-like scary.
You're fairly vulnerable, but as you get more augmentations there are ways to mitigate your chances of encountering bullets, whether physically or otherwise - your choice.
Gameplay is pretty diverse. You can go in gun's blazing or go completely unnoticed. I've opted for the latter because I like the real feelings of tension and suspense it brings. However, you don't need to stick with that one way. You can swap between to two at almost any point in the game.
There are many ways around obstacles such as enemies and door-locks/keypads. For example, I had to get a guy out of a room that was locked. I didn't know the code and didn't want to waste one of my hacking disposables on a side-quest. So I just shot the door, guessing and hoping that the AI would be smart enough to notice sounds outside of the room. The guy and his bodyguard came out to investigate. I downed 'em both non-lethal style.
I definitely recommend this game. I think I've got about 10-15 hours clocked on Steam so far and have only started the process of moving to the second major city.
The above, however, coming from someone who has purchased both DE:HR and RO:2. Both games appear to break the mould. I don't envy the decision OP.
EDIT: I just looked up 'mould' (because it's red squiggly) and every online dictionary says "Chiefly British" - is that not how to spell mould? The alternative doesn't look right to me: 'mold'. With 'mold' I think of the growth you get on a loaf of bread after a week in the sun; with 'mould' I get the distinct definition of shaping or malleating, which is what I intended. Is that just me, or does the 'u' give the word a sense of 'shape'?