Thank God I never have been addicted to any drug (ok, well, I smoked when I was a teen) or been plagued with mental health problems. Whenever I see some person obviously displaying some problem like that (or someone in a wheelchair, disabled, etc.) I think "There, but for the Grace of God, go I."
I think that mental illness would be the worst. At least with addiction, there are successful treatments if you have the gumption to stick with it. With mental illness it's kind of a crapshoot as far as treatment goes.