After writer/artist John Byrne rewrote Superman's origins in the 1986 miniseries The Man of Steel, Metallo was also given an altered backstory. In the current version, John Corben was a small-time con man who was fatally injured in a car crash, but to his luck Professor Emmet Vale happened to pass by. Professor Vale was a pioneer in robotics, and erroneously believed that Superman was the first in a wave of superpowered Kryptonian invaders after recovering Superman's ship and erroneously translating Jor-El's message to his son. Vale transplanted Corben's brain into a robotic body, which was powered by a two-pound chunk of kryptonite, and instructed him to kill Superman. 

  This was Metallo's look for much of the late 80s and was the based design for Metallo later during the 90s Superman animated series. The design by Bryne was perfect, he was having an 80s badass look with fake skin over his robotic body. During most battles with Superman, Corben had his body ripped away piece by piece giving his more robotic look during the height of battle. This was to show just how inhuman Metallo was now and that Superman was not fighting a man anymore.

Underworld Unleashed

Metallo later received a major upgrade via an unholy bargain with the demon Neron. As a result, Metallo was now able to morph his body into any mechanical shape he could imagine (turning his hands into guns or "growing" a jet-pack from his back) and project his consciousness into any technological or metallic device (powers very similar to those of the Cyborg Superman). He could also now grow to monstrous size. During one battle, his gigantic fists were separated and later actually turned into housing by other superheroes. In another incident, Metallo was rendered more insane by the Joker and used his height to destroy an elevated train of commuters.

  Following the story line, the human factor of John Corben was completely removed and all was left was this robotic creature that could adapt and reconfigure his parts at will. The more darker gritty feel of the early to mid 90s worked in Metallo's favor but the story line where his head kept getting saved grew tiresome after so many years. Metallo was no longer a villain with a heart of kryptonite, he was just a evil robot, which took a lot away from the character.

Y2K

Brainiac 13 upgraded Metallo to tap into light spectra and energy frequencies. Metallo also used the technology to upgrade his body to monolithic proportions. This design was based off the Iron Giant movie that was popular at the time, bringing Metallo back to more traditional origin with his heart of kryptonite. This design was very popular but others grew tired of the giant version of Metallo and with the success of the animated series at that time, many wanted Metallo to back to his class look. In Superman/Batman #2 (November 2003), evidence was uncovered that implicated John Corben as the criminal who shot and killed Thomas and Martha Wayne, the parents of Bruce Wayne. This proved to be a ruse by Lex Luthor, orchestrated in order to lure both Batman and Superman into a "final" confrontation. Later , Corben's mind was removed from the Metallo body, and placed into a cloned version of his original human body, by the (second, post-Crisis) Toyman, Hiro Okamura, whose family had invented the metallo alloy. Despite the fact he is once again just a man, he still intended to oppose Superman.

One Year Later

One year after the events of Infinite Crisis Metallo resurfaces, now in a fully human-looking body with an enhanced titanium-alloy frame and plastisteel musculature covered with forced-growth vat-clone organics.  Luthor, needing vast quantities of Kryptonite, incapacitated Metallo and removed his Kryptonite energy core (pulling exactly the same stunt on him as he had before). Luthor upgrades Metallo by placing green, blue, gold, and red kryptonite into Metallo's chest cavity, which he could individually expose at will. Metallo was recruited to be in Lex Luthor's Superman Revenge Squad along with Parasite and Bizarro. After General Zod and his fellow renegade Kryptonians escaped from the Phantom Zone they instead all teamed up with Superman to battle Zod and his minions. Metallo was able to kill six of the evil Kryptonians. Five were killed when he exposed them to power-draining gold kryptonite, while in mid flight and they fell to their deaths and one was exposed to red kryptonite, turning into a giant ant-like creature, and Metallo stomped him to death. Eventually, Superman ripped out all the kryptonite using a lead based battle suit, leaving Metallo without his powers.

  This was more of an updated version of the classic Metallo and adding the various kryptonite colors to his arsenal was a stroke of genius on both writers Kurt Busiek and Geoff Johns parts. Sadly, despite losing his new abilities, it was all a set up for his return during the New Krypton story arc.

Metallo Now

Metallo was later recruited by the U.S. government to destroy the citizens of Kandor after the newly arriving Kryptonians killed five prison guards while breaking the Parasite out of prison so that they could instead place him within the Phantom Zone. Metallo is now portrayed as a normal man wearing a lead alloy armor with only a green kryptonite heart again. He is accompanied by Reactron, a character who is similar to this Metallo, except that he possess a gold kryptonite heart. The pair allow themselves to be captured by the Kandorians and then, while in Kandor, they expose the Kryptonians to their hidden kryptonite hearts and killing several of them, and then escape soon after. Metallo and Reactron later disguise themselves as Kryptonian sleeper agents, in order to capture Nightwing and Flamebird.

  With a new cloned body re-enforced with Metallo alloy around his skeleton and able to channel his kryptonite heart in both wide beams, arm blasts and now having a holographical disquise device built into his suit. Metallo truly returned to his roots and upgraded for a new century. The current design is very close to his original costume and allows Metallo still be a man but keep his cybernetic origins intact. 

Special thanks to Wiki for the added information.