how sinful is marijuana ? does it matter if it is used regularly, abused or in moderation ?
if used occasionally and with moderation(one or two puffs) how is it any different than smoking a cigarette or drinking a beer/wine.
IMHO, it is not a sin like using a hallucinogen (unless it is a powerful grade) because a hallucinogen is a doorway to the spiritual world outside the protection of the Holy Spirit, but I think marijuana is a lot like beer or wine, which must be used carefully.
The question of obeying the law is the more important side. If you consider that having a drug arrest record, even a minor one, will affect you for the rest of your life, then it would be a sin like "gluttony", in which one wrecks one's health for the sake of appetite.
Nowadays even menial jobs are subject to complete background checks, so are rental applications and scholarship applications, and these will turn up even traffic tickets. It would not be good stewardship to wreck your future for the sake of a minor relaxation that you could easily get in some law-abiding way.