Smoking weed can cause symptoms similiar to some forms of schizophrenia, and maybe screw with someones inhibitions (like not stealing things), but you have to smoke a lot of it. I mean, bags and bags of it weekly, all the time.
True life story:
I was once persuaded by a flatmate to let an ex-bandmate of his crash at our apartment for a few weeks. The whole fucking time he was there, his girlfriend, who bought his weed, was there, and the only thing I ever saw them do was smoke pot in my bathroom, eat, sleep, and attempt to be furtive while screwing behind the couch. They did this all day, every fucking day, for as long as they lived there. And when they finally left, I found that my Super Nintendo and my entire collection of SNES games had found there way to the local used game store, along with 3/4 of my CD collection and 3 binders full of DVDs, PS1 games, and PS2 games, all of which I had let him or his girlfriend borrow on one occasion or another. When I asked him if he was done with my stuff (after I bought what was left of it back from the store), he smiled and said he'd have it for me in a tommorow.
So yeah. I don't think weed is nearly as bad for you as cigarettes (expensive, lung cancer, chronic bronchitis, enphesema, smokers cough, 2nd hand smoke, decreased physical fitness, asthma, skin irritation/excema, it stinks, teeth discoloration, bad skin) or alcohol (diabetes, liver cirrosis, high blood pressure/heart failure, addiction, zero impulse control, physical/sexual/emotional/mental/financial abuse, drunk driving, incompetence, ruined careers, mental health/avoidance issues), but it's still pretty fucking awful. And I don't think we should legalize it because it's harmless - it's not - but that we should legalize it because it's about as bad as alcohol and cigarettes, and we don't need to spend trillions more dollars on a prohibition policy for something equivalent to booze and smokes. Also, if we legalize the sale of weed (and maybe a few hallucinagens like igogaine, which is so intense that no one in their right minds would ever want to use it again) within - and only within, no imports or exports - the USA, we'll deligitimize and defund those who peddle weed and hallucinagens along with harder drugs, and perhaps break weeds status as a gateway drug, since it will be normal everyday people who grow it and sell it, not murderous mexican meth lords.
I doubt that will happen though. If we didn't have the weed boogeyman to be afraid of, then we might start wondering why most of the drugs most commonly abused in the USA are perscription drugs.