Those who have smoked it will want it legalised, those who haven't will not care. Therefore it often appears that the general opinion of marijuana leans towards the positive side.
However even though casual marijuana smokers will claim that it has certain benefits (or that "it's not as bad as alcohol"), I don't think that their arguments stand up to the proven mental health risks that arise. Schizophrenia, paranoia, depression, decrease in memory and learning ability, delusions, depersonalisation, bipolar psychosis - and many more.
Therefore I think it's fair enough that it's currently illegal - and based on the facts, I think they should keep it that way.