I don't actually generally think of it as a mental illness, probably because that phrase has so much stigma in our society and I can't shake that so totally avoid thinking about it... it feels like a thing thrown at people to discount their perspectives about what happened to them in the case of ptsd. I guess my experience, and my definition of what happened to me as even being abuse, was pretty thoroughly denied by family for so many years that I'm pretty sensitive to anything that feels similar. If I talk about it to a T or close friend, I'm generally talking about specific problems I'm having or what an abuser did, etc. so the phrase never really comes up.
I think that our brains might really benefit from some help rewiring, but what words to use...? "Injury" isn't quite the right term... I guess "disorder" is decent... "Dysfunction" isn't quite right though I think it's good for various symptoms. "Illness", besides the stigma thing for "mental illness", means viruses or bacteria or genetic chronic things to me... How about "overstressed, sub-optimally organized adaptive neural networks"? Sorry, too much Star Trek.
It seems to me that my brain's networks got stressed beyond what they could cope with in a "normal" way, so I've adapted the best I can; those new brain imaging studies for "overmodulating" ptsd (dissociation-related, as I understand it) and the brain grey matter study shows that different parts of the brain have more grey matter than "normal" people. It's almost closer to a scar in a way...
http://www.ncbi.nlm.nih.gov/pubmed/23113800
People who have had extreme physical injuries to their brains can often recover significantly but may never be as good at certain things as they were before; their lives may or may not be changed, depending upon the injury. I read about Gabriella Giffords, who was amazingly eloquent before being shot in the head (and I think could have been President eventually), that she has recovered somewhat simplified speech through intensive work.... her brain has adapted a lot, but some connections that would be "superhighways" in normal people are more like smaller "side roads" in her brain now. The pathways cannot handle as much traffic. The brain's rewiring ability seems to sometimes achieve different results than "normal development". Gabriella gets tired easily from talking. However she is still a wonderful person, doing great things ... probably more slowly and with a ton of support.