Although Deus Ex Human Revolution is the third game of this series, it's a prequel in time to the first Deus Ex, which is highly decorated as one of the best games ever. The second Deus Ex, Invisible War, fell very short of its predecessor, but this latest game has redeemed the series and is well received by the fans.
This is the story of Adam Jensen from the previous game who works for Sarif Industries as head of its Security Forces. Sarif researches and develops augmentations, which are widely available to those who can afford them. With the world now run more by corporations than by governments, there is plenty of industry espionage with each having their own private armies to protect their developments. In this world, augmentations are the realized invention of their creator, Hugh Darrow, but this has created a split between those who support the freedom to use them and those who believe they are a plague on humanity. It's also created a black market for these augs. To a segment of the population, augmented humans are often reviled as inhuman mutants.
Adam is romantically involved with one of Sarif's top scientists who has made a breakthrough in the advanced biotechnology of augmentations. She is working on the ability to link the human brain with mechanical implants. Other groups are vastly interested in this and to such an end, Sarif is attacked and this research division destroyed. In the process, Adam's girlfriend is killed and Adam very nearly so. However, he is saved by the science of augmentations. Six months later he is back on the job and looking for answers.
I seem to be getting surprised lately with some of these AAA games and this was no exception when I first saw the character renditions. It can only be assumed, of course, that its visual style was intentional, but if the developer's aim was to re-capture the glory days of the original Deus Ex, it was a bad decision. The renditions are just awful and the cut scenes even worse. So why, you might ask, does it get good reviews? Simply put, the gameplay far outweighs the visual presentation and not all of the presentation is bad. In other words, the outdated visuals are rendered graphically well, if you can make sense of that.
As with the original Deus Ex, stealth is a really big factor in this game. Although combat is one optional way to play, it's far too easy to get killed taking on the enemy head first. In this way, the third game returns more to the roots of the original by forcing choices on what to carry and what to augment. Inventory space is once again at a premium and combat can be restricted due to low weapon ammo capacity. However, weapons can be upgraded by buying or finding upgrade kits. The game encourages exploration to find these as well as rewarding you with experience points for scouring locations.
Hacking also returns and gains you the most experience points. I disliked this part of the game as it is used so heavily and is too convoluted to fully understand even with the tutorial. Basically though, you must capture all the nodes before the network detects you and sets off an alarm. In higher security situations, this can only be accomplished by using worm and virus software you can find or buy. Generally, in the must have situations in order to progress, hacking has a low security rating so you shouldn't get stuck. Sometimes, there is a bypass by using vents to reach a location, but not always. A must have augmentation is “hacking capture” to at least level 5.
Once you get over the shock of the dated visuals and the frustration of understanding the hacking, you can proceed to enjoy this game. Stealth plays a major role and the game is weighted in favor of this approach, although combat always remains an option. There doesn't seem to be any penalties for using both unless you are going for an achievement. If you are going for a no-kill game, the bosses that cannot be killed by stealth, don't count against you. If you are having trouble with them, the first two can easily be killed with the stun gun and the last with the laser rifle or grenade launcher. In fact, the bosses are a bit of a joke once you use the right weapon on them. The game is not particularly hard, but I would say that combat is a harder way to go than stealth.
I experienced several crashes, freezing, and stop motion problems, so check the section Game issues for the solution. After applying the fix, my game ran smooth as butter. The interfaces are good with a drag and drop inventory and logs etc and are all easy to use and understand.
The Missing Link DLC tells the story from the time Adam enters the stasis chamber till he emerges from it in Singapore. It assumes that you've played Human Revolution, so it doesn't waste time getting to experienced play. This DLC is launched seperately and your updated augmentations and inventory are not carried over from the main game.
As far as choices on what to update, I found the DLC far more leading due to the circumstances dictating how to spend your praxis points. It is not recommended that you play this till at least reaching this point in the main story, but as it does not effect anything in the main game, I just played it afterwards. I would definitely recommend this DLC.
UPDATE MAY 2015: A Directors Cut version of Deus Ex Human Revolution was released in Oct 2013, which apparently fixes the boss fights, but earlier releases cannot be upgraded to this. There is no patch to fix the original game and nor will there be one. The DLC, Missing Link, is integrated into the main game of the new version. You should specifically research this version as it appears it had many bugs at the time.