not sure id call it obsession/addiction... perhaps 'focus' is a better term. when im working on something (3D/leather/sculpting/gunsmithing/whatever) im really driven ... to the point where i dont eat/sleep/talk until i finish. im like a zombie. its pretty brutal in a relationship, cause you are in this work trance and your SO inevitably takes it personally that youre being a douchebag and not paying attention to them.
conversely, i get burnt out easily too.  if i am trying to get something to look/work exactly the way i want it but its just not happening because of the program/medium's limitations, then i get frustrated and discard it.  the few Poser figures ive released are like the tip of a huge iceberg of work-in-progress that never sees the light of day.

as for substance addiction, im strangely non-susceptible. ill smoke for a few weeks, then not even think about tobacco for years. ill be a hardcore alcoholic for a few weeks then not touch alcohol for a year or more.

and sanity? - its overrated. all the sane people i know are boring as fuck. sanity is the playground of the unimaginative :)