The Skull Knight or another person succeeding his use of the Armour didn't stop fighting until all of his blood was spilled and all of his bones were destroyed, pierced all over by the Armor before meeting their end.
It can also cause temporary insanity, causing the wearer to enter a "berserker rage", it was also shown to increase fear as when Guts used it to fight the Makara and was a little in control he felt that the ocean waves are swallowing him and start to see monsters surrounding while it was merely the alligators who were no longer being controlled and Guts attacked them out of fear, and rendering them incapable of distinguishing friend from foe. In the long term, it can cause chronic mental deterioration (such as the strengthening of Guts' Inner Beast and severe physical effects as well, some of which may be permanent, such as the loss of senses.
It is believed that prolonged use of the Berserker Armor is what caused the Skull Knight to enter his current skeletal form. The Berserker Armor was originally created by Dwarves and it is hinted that there are more than one armor as Schierke said that the Dwarves liked to make this type of armors and Dhaiva recognized the armor as the Plana of Durga, which mean that the Kushan encountered such armor in the past.
It is very durable as it withstood direct attacks from forceful beings such as Grunbeld in his apostle form, and the Makara with out suffering a dent but it doesn't guarantee the safety of the wearer as the force from such impacts can break and shatter bones, also piercing attacks prove to be more effective as it can make holes in the Armour but it should come from something powerful (e.g. Makara's bite and the Sea God's predatory fish's bite) after some time the holes made on the Armour start to close or "heal" due to it being somewhat magical.
After being used by the Skull Knight, it was kept by the witch Flora for a number of years (possibly centuries or longer) before being given to Guts when she was under attack by a group of Apostles under orders from Griffith to kill her.
The witch apprentice Schierke has demonstrated the ability to help Guts retain his sanity while under the influence of the Armor, although this ability is far from foolproof as the moment Guts' heart is possessed by hatred the Hellhound immediately takes over (while carrying Casca and Farnese, he indirectly squeezes them). In recent chapters, it appears as long as she is in astral contact with Guts prior to the use of the Berserker Armor, Guts is able to maintain his sanity. Schierke has pulled Guts back to sanity three times, but with assistance from Guts' child for two of them. As long as Guts' suppresses his inner demon, it will continue to grow in strength. Schierke was barely able to pull Guts' back to his sanity the last time as Guts' inner demon resisted the pull back to sanity.
There appears to be a distinction in power between the uncontrolled berserker state and the controlled one, Dhaiva hints: "but could you defeat me by losing madness?" meaning the uncontrolled state charges directly with any form of tactics and the use of power is at its peak hence making him very agile by constantly doing acrobatic attacks several times while airborne. The state in which Schierke keeps check of Guts' sanity (piggybacks and scrapes the covering off his left face) is him augmented.
The Armor takes on the appearance of the wearer's "inner beast" when entering the Berserker mode (Hellhound for Guts), though part of the reason could be that Guts' inner beast flowed from his ethereal wound into the Armor and changed its shape. In this form, Guts easily cuts through hordes of Apostle and Kushan monsters. Guts' prosthetic hand can function (in that the fingers and wrist move) like a real one, furthermore fire the cannon without the need to pull the trigger (the hand Armour lifts up while the prosthetic one drops down) when he is under the influence of the Armor.