There has been a lot of talk about Geoff Johns using his coined "geoffcon" with well established characters. While some have been enjoyed, others have been frowned upon. One that seems to cause a bit of debate is having Metallo going back to a human/cyborg in a suit with his heart of kryptonite. After Corben comes to Metropolis to visit Lois alongside Lane and is rebuked by her, Corben volunteers for a prototype of a large power lifter type exoskeleton developed by Lex Luthor for the US Military. The exoskeleton is coated in a special alloy developed by Emmet Vale, called Metallo. The suit is powered by a chunk of Kryptonite.
When the US Military assaults Superman in the Daily Planet building, Superman's laser vision strikes the armor, sending shards of Kryptonite into Corben's heart. Sam Lane asks Luthor to operate on Corben. Luthor agrees, transplanting a chunk of Kryptonite for his heart, as well as giving him a smaller suit of armor. Superman defeats the fledgling Metallo by welding a lead manhole cover to his chest, protecting him from the Kryptonite Radiation, and flying him into the upper atmosphere, as Corben still has to breathe oxygen to function.
The truth was, this was how people knew about Metallo for close to 20 years before John Bryne redid his origins. Metallo was able to escape using a back up power cell. He sold his soul to the demon Neron for more power and soon after, joined forces with Braniac 13.
Corben's metallic body grew to giant proportions after absorbing Metropolis technological infrastructure. Metallo's body eventually downgraded to a more human size and he was also able to shape-shift his body into nearly any mechanical form imaginable. The truth was, it was a very good idea to update Metallo to his more robotic structure, which became the villains status for another 20 years. However, the truth was, he started becoming this giant robot that once was a man and had lost his humanity. The ball got rolling just before Infinite Crisis where John got a new human body that was cloned from his original but missing his heart once again for kryptonite. This took the character back to his roots and following "Up, Up and Away" the more traditional Metallo, a former man who was turned more cybernetic than human from his encounter with Superman took it's place again and it grounded the character again. Metallo being a man who has lost that part of his humanity thanks to Superman and Corbon's own ego makes him easier to identify with and makes him a stronger villain as a result. In this case, a man in a metallic suit with a heart of kryptonite where his heart used to be works very well and keeps him on top of one of Superman's greatest villains.