A very strong entry in the franchise that is just missing a bit on a few bits here and there that hold it back.
First off, Human Revolution is a prequel, and outside of some inside references to events and people in the original Deus Ex, there is very little connection between the two games other than general world and theme. So, don't feel intimidated by it being the 3rd in the franchise, it can easily be played as a stand alone with full understanding of the plot and mechanics.
Now, as far as mechanics goes, this game is a mix of stealth and action, which is not surprisingly compared to Metal Gear Solid. Where this differs from MGS most is in the aug system, were as you accomplish various deeds you gain experience which can be used to activate various enhanced abilities your character can use. Augmentation can ease up play, or open whole new paths for you. Overall its a pretty fun system, and by the end you will likely have pretty much every aug you could want.
Another big aspect of this franchise is the notion of choice of approach. I actually think Human Revolution got this even better than the original. While run and gun is out, an offensive approach is viable throughout, as is a stealthy approach, and a good mix of the two. Whole areas can sometimes be made much easier through a conversation battle being won which is also great. Even within a styles, there is often lots of routes to proceed forward.
Story wise, for the bulk of the game, it is pretty good. It touches on a real life issue we will likely be facing in the not so distant future, and doesn't make the mistake of taking any particular side. They is a fair amount of dialog and characters that all work fairly well, and a huge amount of written content to flesh out the world.
Now, that's the good, onto where things missed their marks. The aug system, much like both of the other Deus Ex games, is not well balanced. Some augs are clearly better than others, and a few are downright useless, or near to it. Another problem is the battery system. Only your first battery will recharge if empty. All others only recharge only if they have some charge left. However, many augment actions take 1 entire battery. This system really devalues expanding your batteries, and for most the game you're best advised to simply run off only 1 battery. Now this is there for balance reasons, but one can't help but think they could have done this better.
Also, while choice in approach is diverse, it isn't perfect. The aggressive approach is less experience rewarding, and again, much like the original, there really is a best way to do things, which stealth nonlethal. Still this is a fairly minor thing. Probably a bigger issue is the choice in story. There are times when you get choices, but compared to heavy choice based games these are rather limited and shallow.
Probably the biggest flaw, story wise, is the last bit of the game. It does feel rushed. I won't reveal anything, but there is definitely the impression that this last part of the game was done under the wire.
Beyond that, graphics wise it isn't going to win any awards, the gun play isn't up there with more dedicated shooting games (although still perfectly manageable), and there is a few technical hick ups.
All and all, it is still a very good game. Just don't go into it expecting a perfect shooter, or perfect stealth game, or a deep rpg. It is an amalgam of all these things.