Kyoroto

OK, first I am going to avoid spoilers throughout this review, so I would like those who want to comment to avoid the same, even in spoiler boxes.  This is one of those games that is going to be harder to enjoy if you spoil it for those who come after, so I will personally eat anyone who spoils it.  This VN is by the same writer who did Sumaga (blech), Gekkou no Carnevale (yay), and Totono (a VN a lot of people here liked).  However, the style of story is much closer to that of Gekkou no Carnevale than to Sumaga, thankfully.  So, those of you who have heard me bitching about Sumaga need not worry that this is going to turn into a long rant on how horrible anything by this writer is.  First, I am going to give you an idea of the basic setting.
It is 2199 and the world has plunged into a new ice age (this is actually one of the hypothetical results of global warming irl, if the Greenland glacier slides into the ocean early enough).  Up until a decade before the story, humanity was fighting over the warmest areas of the world, putting all their power into a war that was becoming increasingly meaningless.  In that war, there were many scientific advances... but the two that are most relevant to the story are Necromancy and the zombie-killing techniques developed by the protagonists' fathers.  Necromancy is undertaken by injecting a special type of nanomachines into the human brain after death, turning the individual in question into one of the Living Dead and the person who did the injecting into a Necromancer, capable of moving the undead results at will.  The zombie-killing techniques central to the story are the use of one vs many weapon techniques (the basic one is two pistols with muzzle spikes, which the male protagonist uses, though the female protagonist's basic fighting style is based off of the same) in combination with the EX-Brain, an analytical computer that fits onto a person's head and allows them to notice things their subconscious recognizes but their conscious mind doesn't (details the conscious mind filters out), thus allowing the person in question to fight more effectively.  Tokyo of the future is a dystopian hell, where refugees from all over the world have gathered, necromancers roam the streets with their armies of the living dead, and bounty hunters (like the protagonists) hunt them for their daily bread.  The city itself is kept alive by geothermal heat spread throughout the city by water-filled 'hot pipes' passed through the hot spot underground and circulated throughout the city. 
Main Characters (protagonists, heroines, and main antagonist)

Nagaoka Souun
Souun is the son of a hero of the American-Chinese War, who was killed some years ago.  On the surface he doesn't show much in the way of emotion, and his fighting style is one where he uses the dual-pistols in combination with his EX-Brain to fight in the most efficient way possible, obeying the suggestions of his own subconscious like a machine.  Indeed, he himself isn't really sure he has emotion, and that is one of his major internal conflicts throughout the story as he deals with his personal issues.  He is a badass, to put it bluntly... the kind of guy who can re-kill thirty zombies in under a minute without taking a single scratch.  His body has been trained using both conventional methods and tailor-made nano-drugs to turn it into a weapon capable of perfectly obeying the EX-Brain's suggestions.

Kibanohara Echika
Echika is in just about every way Souun's opposite.   She is a rebel, preferring to use a chainsaw and shotgun and frequently ignoring her EX-Brain's suggestions in favor of her own impulses.  She believes emotions should be immediately be shown on the surface and indulged, and she is more than a little hedonistic by nature.  She is also a straight-out lesbian who has no hesitation going after pretty girls.  Her outright hatred for her father is the most obvious cause for her rebellious nature and her unwillingness to use her EX-Brain (the horns, as opposed to the full helmet Souun version Souun uses). 

Hougyou Iria
Iria is the main heroine of the story and the focus for most of the events in it.  She has extreme synesthesia (look it up), though she is quite functional despite that.  Her initial setting, amnesia, is one of the most obvious tropes out there, but it isn't the focus of most of the story.  In a world where people are rapidly losing their emotions, both positive and negative, she is a bright, cheerful young woman whose very presence makes people feel hopeful and light-hearted.  She is also an otaku. 

Kijou Mitsumi
Kijou Mitsumi is Souun's 'other' potential heroine.  She is, like him, a hunter of the undead.  However, she uses a katana and muscle-enhancing suit, along with her talent for mapping out the actions of enemies on the battlefield, rather than the style used by Souun and partially rejected by Echika.  To be honest, this is one girl who has a singular talent for getting herself into trouble... and a lot of it is caused by her somewhat single-minded personality.  She is very devoted to any cause or person she latches onto, but that also means she tends to become dependent on those she attaches herself to, which is the cause of her personal conflict.

Aso Kiriri
Echika's  adoptive older sister.   She is the most 'normal' person amongst the main characters, ironically, despite the fact that she is a high-ranking member of the Military Police under Echika's father.  She is kind-hearted and constantly worries about the broken relationship between Echika and her father, as she loves them both.  She also is very idealistic, believing that her duty as an officer of the law is the protection of Tokyo's people above all other things (an idealism almost unheard of in the somewhat tyrannical Military Police organization). 

Kon Su
Kon Su... is probably the straight-out weirdest character in the VN.  She is a hard M (with bold and italics for emphasis), and she has casual sexual relations with both protagonists.  She is a professional hacker, information broker, undead analyzer (dissects and analyzes undead bodies and brains to discover the individual styles of necromancers... sort of like forensics), and she is also one of Souun's and Echika's supporters.  Unfortunately, explaining her weirdness is not really possible without spoiling the VN, so I'm just going to drop it.  Incidentally, her path isn't really a straight-out heroine path, for reasons that will be obvious to anyone who completes one of the paths.

Milgram
Milgram is the charismatic leader of the Researchers, an organization of Necromancers that believe the only hope for humanity is to escape from emotion through death (extinction).  He isn't your standard nihilist, but the philosophy he espouses is.  This guy... is the single most powerful Necromancer in existence, which means he is also an extremely emotional person (emotion and artistic sensitivity being requirements for creating the best undead).  This guy is flat-out scary, eerie, and freaky.  However, in many ways he is a representative of one of the major aspects of the world the characters are living in.
The Story
For better or worse, this story has a ton of guro and is basically an action-horror story.  If you can't deal with heroine deaths, torture, zombification, and occasional rape, this definitely will be hard on you.  A lot of effort goes into portraying the most gruesome parts of the story in a way that will be as horrifying as possible, and the heroines don't always escape this particular treatment, so it is best to prepare yourself before jumping in.  The lighter points - mostly centered around Iria - actually only serve to enhance the sheer horror and bleakness of the setting, which is basically an extension and enhancement of what this writer did in Gekkou no Carnevale. 
By nature, any story as full of zombies like this one is going to be dark, and this one really does go to the extremes of the dark end at times.  It is really hard to talk about this story without spoiling some of the primary elements (and because I figured out those elements too early due to certain hints, my own reactions were more ones of 'expected horror/sadness', so I'd prefer not to do that to you all).  To be honest, emotionally connecting with this story was hard at some points, but as I got deeper into the characters - usually near the ends of the paths - it became a lot easier.  In terms of action, this VN is full of it, but it is most emotional toward the end of the paths and in the true route.
Kon Su's route serves a purpose different from the other routes, revealing elements of her past that are important to the story as a whole while showing the steel that lies beneath the stuttering, drooling do-M surface of that character.  Whereas the other routes have some element of romance, Kon Su's lacks that, and I honestly thought that was for the best (I honestly couldn't imagine any ending where she was romantically involved with either of the protagonists, as opposed to just sexually involved).  Ironically, if I were to say which of the routes (other than the true one) touched me in the most positive manner, it would be this one.
I really suggest anyone avoid playing Iria's route until you've played all the other heroine routes.  It feels a lot more natural if you go into the true route straight after finishing her route, as the true route is in many ways wrapped up the most intimately with hers.  The other routes can probably be played in any order without a problem, but I do advise you take my suggestion above seriously.
Visuals
Normally, I don't talk about a VN's visuals that much, but because of the styles used here, it needs to be mentioned.  First, I should mention the action-scene styles.  The action-scenes are defined by a type of cell-shaded animation that is very similar to SMT: Digital Devil Saga on the PS2.  Literally, the action scenes are animated and combined with highly-detailed battle descriptions to give a degree of depth that is pretty rare even in action VNs.  I was doubtful at first, but this VN definitely benefited from using this particular technology (though it probably detonated a nuclear bomb under the budget).
The regular visuals, as seen above, are more 'classic-style' Nitroplus, so if you've played a Nitroplus game made since Muramasa, you probably noticed that it is in the same general style.  As a result, there is no real need for me to discuss them... except that the way the designers used them was pretty amazing.  The dystopian feel of the pipe-wrapped city of Tokyo in 2199 is pretty eerie-feeling and definitely adds to the general atmosphere of the story.  The tendency to dress most of the characters in dark or harsh colors (except Iria) was probably intentional, to further add to this atmosphere.  This is one of the few times in the last year when I've actually felt that a company went above and beyond when designing every visual aspect of a VN.
Audio
Musically, this game isn't really unique.  The BGMs all feel 'familiar', though they are used effectively to enhance the mood, so I give this VN high ratings for its BGM use, if not for the songs themselves.  Nitroplus's use of music shows a tendency to prefer unobtrusiveness and 'enhancement' as opposed to the use of music to 'define' the mood seen in a lot of other VNs (Hapymaher being one of the most extreme examples of the latter). 
What really struck me is the wide variety of sound-effects, such as gunfire, cutting sounds, etc, and the use of those sound effects.  To be honest, the sound of zombies being blown apart by bullets in this VN is going to linger in my ears for quite a while, as is the sound of Echika's 'Rabbit Punch' chainsaw. 
Overall
Overall, this VN is pretty impressive.  I can honestly recommend this... though not to just anyone.  To be honest, the 'average' VN-reader who prefers moege-variants will probably not be able to stand the darkness of this game.  If you don't have a reasonably high tolerance for guro and dark atmospheres, this is going to be a hard VN for you to enjoy.  Zombie-lovers will probably flock to this VN by the thousands if it ever gets translated, because it really does draw in a lot of what people like about the 'zombie apocalypse' style movies and TV shows, while giving it a uniquely Japanese/otaku media flavor.  However, this VN is emotionally draining, so I do recommend taking it in smaller doses than I did (every minute I wasn't working for the past four days).  In my opinion, this VN can be considered a straight-out kamige, but it is also a VN that picks its readers... simply because it is so high-stress.