eXistenZ is an odd movie. First of all look at the name. Take it all in, and be reminded that all the capitalization in the name is correct. Just looking at the name you expect something that is fucking weird, and it delivers, it delivers in spades. Remember Inception? Well it's kind of like that for games and not a bad movie. There are less pointless action sequences and it all makes some sense within the world that is clearly not our own. Also unlike most game movies that kind of ignore or most of the time insult their audience eXistenZ doesn't do any of those things because it presents a very different gaming experience, one that doesn't exist but is possible in the foreseeable future.
The main device of the film, that of a game that puts you in a complete alternate reality is not unknown. Ignoring the videogame portion of it the concept of an alternate reality is fairly common in science fiction, though less common in Canadian horrorish films. The film is not a horror in that there is no real villain and there is a decent amount of comedy but a lot of the imagery associated with the film is relatively horrific, including the game controller itself. The controller is effectively a blob of flesh with a few fleshy bulges on it that functions as buttons. They also all come with umbilical cords that plug directly into people's backs in a strangely sexual way, ie penetration and lube are both mentioned and shown.
Back vaginas is what I'm trying to say. But back to the actual controller ports and not the weird ports on humans the things are a hell of a lot more biological than it appears at first glance, actually having organs and bleeding when one is dissected later in the film. Essentially they are more living than most of the insects that buzz around, and the way the main character treats it the controller is more similar to a pet like a cat or dog than anything else. This weird relationship is further heightened by the way people talk about the game, as if it is a need more than a want. Effectively the game that has been created is a drug and the female protagonist is the cook.
There is something about her eyes that just betray the pangs of addiction as she pleads to be let back in to the fantasy world that she has spent five years working on, and just needs the chance to play. That need is what indicates that it is something more than just a game, and even more than something like World of Warcraft is nowadays with people literally dying playing. If something like this was created in modern day there would be a huge number of deaths as people ignored all their needs to play the game. A time distillation effect is mentioned but considering how dedicated some Otaku are it seems likely they would still manage to starve or die of thirst before returning to the real world which seems 'less real' than the game after playing it for only a few hours.
eXistenZ shows what happens when something like this is created, a game which can replace reality. Sure there are games like the Sims and Second Life but an AR game like the eponymous one is so clearly different for one crucial reason. There is no user interface, or controller, or anything to distract the player from the game. Even if they feel the hunger pains they will ignore them to explore a magical world with no consequences beyond resetting the game, a world where they can do anything. It might cut down on crime even because a player could murder and steal and do everything they wanted and have it feel real, but it wouldn't hurt anyone in 'real' life.
Of course if a technology as advanced as this was created the question of what is reality would be brought up frequently, and debates would rage over whether is was suitable to use with children and whether it was harming the populace who used it. There are always debates, but as shown in the movie this kind of technology might cause a further divide. Not only would people get angry about it and make an internet post, but people might actually resort to murder to destroy these kind of things before they became mass produced, before everyone began to descend into their own fantasies about good and bad and everything that is fucked up about the world.
The ending has a sort of Matrixy feel to it as there is the question of whether or not reality is just a game, is it all just layers on layers of falsities and if it is does it matter? The question is not answered and it ends on that note, one of confusion and doubt because honestly there is no proper answer. Sure, I believe that in the film the final scene is in reality, but if one can be created that feels so real than what is the advantage of one over another? That is the debate that would rage and it is certain that some people would descend into these games and never came out, no matter who it hurt including themselves. It could be the best thing in the world to live a virtual world where anything is possible and there are no consequences, ever.