A few additions:
Willk has a very solid answer. I maybe disagree on a couple minor points.
Rubbery skin ISN'T skin made of rubber. I've got to go with John on this one, rubbery skin will not be as good at keeping armored plates in place, or resisting the complex crushing/shear forces of blunt trauma (there are reasons land animals don't have rubber skin) or the armor-piercing slashes from pole arms like halberds. Octopus skin isn't really going to be a good solution for something on land being stabbed/slashed/shot. I'd go leathery.
Otherwise I'm figuring the armor is akin to medieval armor, but with more superficial injury to the outer dermis than actual armor would normally allow. I'm not sure how good it would be vs bullets (chain/plate didn't fare well). IT WILL BE HARD TO FIX BUILT-IN ARMOR. Once injured, the armor won't join together again (since it sounds like these are implants). If the armor ISN'T implants, but grown, it will be a lot more likely to be a version of bone (bone IS metal-based already); at that point it's indistinguishable from osteoderm. They'd have decreased agility in all situations due to obligate weight. Overall, I don't know that it would be BETTER than wearing a good suit of plate/mail armor, except it would be hard to take away, and the creature would be always ready.