If it was legal it could be controlled in a way similar to alcohol. Might stop people driving while stoned anyway. The number of friends I have who think it's ok to get into a car and drive after a few spliffs beggars belief.
Using it with mental illness is a bit of a gamble. Some claim it helps; I know of plenty more who developed problems as a result of using it (mainly paranoia but also depression, and a few went on to develop psychoses).
I also have one really good friend who has MS and really does get some relief from smoking it.
It's one of those things where there is no easy answer. Making it legal means at least there'd be control, of quality too, especially if it was legalised for medical use.
But then I think taxes. You can guarantee it'd be taxed to high Heaven.
I'm not sure - it certainly does a lot of harm to a lot of people (not least because it's smoked, which damages the lungs - I'd like to see a pot-pill available

) but it can also be beneficial.
I don't honestly see much difference between takling chemical medications for mental illness and somethign like marijuana. They're both a gamble, they both have the potential to cause more damage than they fix. I don't get people who say 'don't take the meds' but that it's ok to smoke.

But if you're happy with medication, and you're ok with smoking (or eating it, whatever) then if it helps, it helps. Be nice to have that help without breaking the law.