The following paragraph of information is from the Gundam Project website (gundamproject.com), which has a TON of technical info about anything Gundam that you would ever want. I highly recommend you go there if you even have the slightest bit of interest in finding out how these machines work...it's fascinating.

"The most dangerous feature of the Wing Gundam Zero, however, is its unique Zero System. This experimental control system interfaces with the human brain, transmitting combat data, tactical analysis - even predictions of future events! - directly into the pilot's mind. While it bestows almost superhuman reaction speed and fighting ability, the Zero System has the potential to override the human pilot's judgement and distort his perceptions, turning him into a berserk killing machine."

Basically the Zero system is a mind control device. It integrates the living pilot and the Gundam machine into one single fighting unit, as opposed to the pilot and the Gundam as seperate units which complement each other. When this happens, the pilot is able to BECOME the Gundam and "think like a Gundam," if you will. The readouts from the Gundam's systems, the CMD, the radar, the positioning system, the weapons controls, the HUD...all this will be transmitted directly to the pilot's brain without him physically having to process this information secondhand. Vice versa, his brainwaves will transmit directly to the control systems of the Gundam, and this way his response time is quicker (as opposed to the brainwaves having to travel through his nervous system for him to hit the controls), his fighting style more precise and lethal, and as a PART of the Gundam, he will be capable of maneuvering feats that a normal pilot would never even consider. Thus the "almost superhuman reaction time and fighting ability."

This is where the Zero system's "perfection" goes awry. There are a few things here that render the Zero system inoperable by a human being. The human body is designed to take only a certain strain on the mind, a certain number of g's, a certain level of sensory experience. The Zero system is sensory overload. Gigabytes of information are coming through the control circuits at once, and the pilot is human, not a machine. The Gundam is a computer, designed to process information faster than any human brain ever could. Ever had a computer class where the professor compares the speed of the human mind to the processing speed of a computer? The difference is enormous. There is no way the pilot's brain will be able to handle the processing strain for a long period of time. I'm not sure how long an average flight time was for the pilots in GW, but long term exposure to the Zero system would, in my opinion, cause brain damage.

The second danger of the Zero system is that it gives the pilot a sense of invincibility. Being a machine, the Zero system does not fear death. It has no emotion, no sensory perception. It can take 13 g's with ease and "live" another day. When it is destroyed, it feels no pain. The "whisper" of the Zero system to the pilot as it transmit directly into the pilot's mind is that the PILOT himself, now as a part of the Gundam, is also invincible. This is the distortion of perception the paragraph talked about. The pilot has no idea of what his body is capable of doing, because he now is trapped in the machine mind of the Zero system. It would be very easy for the pilot to kill himself simply by performing maneuvers very possible in a Gundam but impossible for the human body, and not even realize it.

Finally, the "bersek killing machine" is a combination of the two above points. The sensory overload plus a feeling of invincibility warps the pilot's mind, perhaps irreparably. Hooked directly to the machine mind of the Gundam, which is in itself a killing machine, the pilot becomes a killing machine, killing with both the precision of a machine and the emotions of a human. Not only that, but these emotions have been magnified due to the warping power of the Zero system.

Now onto the effect of the Zero system on each of the Gundam pilots. The Zero system's effect on the human mind can best be described as a "deadly wish fulfillment." It acts on the mind to bring its strongest emotions to the fore. It'll give you what you want, but it will break you in the process. It gives and takes at the same time, and what it takes it never gives back. The Zero system also appears to act accordingly to what kind of emotional control the pilot has placed on his or her mind. Heero is usually referred to as the only pilot who has ever mastered the Zero system. The fact that he was able to control Wing Zero is proabably partly due to the fact that he has an amazing emotional control ability. Duo, as a very passionate and emotional person, wwas not able to handle the Zero system. Wufei's mind, on the other hand, thinks in terms of logic, so he was able to stop the Zero system (not necessary control it, but at least stop it) but digging through the reams of information the system was showing him and ferreting out what he wanted to see (the Gundams, Zechs, Treize).

The Zero system gave Trowa back his memories. Why? Trowa isn't like Duo, with strong emotions, but he's not like Heero either, who has repressed all his emotions. Trowa's confusion about his identity and why he was fighting was brought to the fore when he was using the Zero system, and the system, before it could fully integrate itself with him, had to establish a firm connection with his mind. In order to do this, it had to stimulate his brain to bring back his memories. The Zero system cannot act on a mind which does not have a firm grasp of its own abilities and limitations, because what the Zero system does is magnify those abilities and decrease limitations. Trowa, not knowing what he was actually capable of because he had lost his memory, could not use the Zero system until he regained full control over his mind.

Lastly, Quatre. Some people disagree that Heero was the only person to "master" the Zero system, and that Quatre handled it admirably as well. How is this possible? Quatre doesn't have Heero's emotional control, nor does he have Trowa's amnesia.

In the original Mobile Suit Gundam universe, (not the spinoff AU GW universe) there exist humans with psionic abilities, called "newtypes." In GW, we don't have these people with the psionic newtype abilities, but we do see shadows of these abilities popping up. One example is Quatre. His "uchuu no kokoro" is actually the closest GW gets to the original Gundam newtype ability. It would make sense that Quatre was able to "master" the Zero system because of his newtype abilities. Since newtype abilities are based on brainwave and manipulation, Quatre's far more developed mind would probably have enabled him to use the Zero system with more ease than a normal human such as Heero or Zechs.

So what about the Zero system in other series and the real world? As for other anime, there is a very similar scenario from Macross Plus. The Macross Plus fighter is called the YF-21, a prototype fighter which is controlled by directly linking the pilot's mind to the ship's computer, with no external controls whatsoever. Brainwaves are the only means of powering the craft and using its weapons systems. The danger that the YF-21 faced was that as its pilot's emotions changed, its control became more and more erratic. The control system would take the pilot (Guld's) emotions and increase them to the point where he would become completely out of control, very much like the pilots of the Zero system would. (It's also my belief that the design for the Zero system was partly based on the design for the YF-21, since the design team for the Gundam series contains some of the people who also worked on Macross) Basically, the YF-21 control system is a slightly toned-down version of the Zero system.

It is my theory that most shounen mecha anime have at least one "version" of the Zero system somewhere (EVA and Escaflowne have versions of it as well). It's a big debate in the real military today, which obviously has to have some outlet in the anime world, especially with anime such as Gundam which deals with political and military issues.

There is actually something like this being developed in the military today. It is called the HCI, or Human-Computer Interface, and what the HCI does is use human brainwaves and facial expressions to eliminate the use of extraneous movements of other body parts. For example, maintenance crews for B-52 bombers, instead of having to memorize fifty manuals of information, could simply go out to the flightline wearing a HCI computer attached to their belt and a sort of headset which could display information relevant to what the maintenance worker wanted to work on at any particular moment, just by thinking. That would certainly make maintainance more efficient, and one of the things the military prizes is efficiency. The military is also thinking about starting to conduct "emotional training" to see if pilots can handle their emotions well enough to incorporate the HCI into piloting systems...a real-world YF-21, if you will.

So in light of this, what is the line between efficiency and mind control? What turns a YF-21 into a Zero system? When should humans pull back and say that there are some things better left to machines alone? Heero Yuy before the Zero system was already a machine, a living, breathing computer. His constant use of his laptop symbolizes this. If Heero is the Perfect Soldier, what's to stop humans from turning into machines, anyway, if that's the perfect way to wage a war?

To conclude, the Zero system is not a toy, and it's a wonder that none of the Gundam pilots who ever operated it were killed or were driven completely insane.