My personal anecdotal evidence is that I can't even go to a rock concert without having to taking clonazepam beforehand and immediately heading to the bar for booze, just to prevent the serious crazy that happens if I even get a whiff of second-hand weed smoking. That's right, even a contact high fucks me horribly. However, on the other hand, I know scores of people who smoke the stuff regularly, either for recreational reasons or medicine, and either benefit or have neutral effects. It's a drug, and like other drugs, it causes varying reactions amongst various people.
I think it should be legal, but I don't know whether it's better to regulate it like medicine, or like other recreational drugs like alcohol or tobacco, both of which are more harmful.