I too was intrigued by this question and just joined this site. My personal experiences with cannabis have all been positive. I was a moderate smoker in my late teens. I was one who stated I would never touch drugs. Until.... the anxiety/ocd/depression came and hit hard years before. When I got to a point that I just didn't care about life or anything, I tried it. I had someone who was experienced with it at the time let me know NOT to smoke more than a few puffs. I believe that is why lots of people get paranoid heavily their first few times, or first time, and have a bad experience. They quite possibly smoke too much.
Anyway, my anxiety went from a 10 down to a 4 or so, depression lifted away, it was great for me. Too bad it's illegal in so many states still. It can definitely help some people. That has been proven. But as stated above, it is not a front line medicine. Plus the type indica/sativa can play a decent role in how you react.