i have slight tinnitus, when i have my iems in (without music playing) or its very quiet, i have a soft ringing in my right ear and a very very soft ringing in my left ear. Its easy to ignore, but if it got alot worse i could see it becoming a major problem.
My uncle had severe tinnitus caused by listening to rock music when he was young (literally lying down with a speaker on either side of his head cranking at full blast...). He said his hearing gradually deteriorated and then his tinnitus became really bad, he couldnt sleep, he couldnt work, he said that he came close to killing himself because the loud ringing was driving him mad. He said he felt like he was actually going insane.
Its a story which was enough to scare me off listening at loud volumes. I think i caused my tinnitus by using my sharp md33's waaaay too loud, and also due to raves + car stereo going nutz when i was at uni. Now i make sure i take precautions, nothing crazy but i'll definitely wear ear plugs to concerts/live events/clubs. and i listen at low volumes.