There's way too much to say that you don't know about it that you need to know. Look for some documentaries, a good one to start with is The Union, but there are many out there.
It helps with muscle pain, MS, cancer, etc. If it were legal and used as a medicine, 10-20% of the drugs on the market would be unnecessary as they would be treated with Marijuana. The side effects are: you either get really high or really stoned depending on the strain; you are hungry; you laugh; everything seems better; it is an aphrodesiac...
It's never killed anybody in it's 10,000 years of use, it's not bad for your health at all if you vaporize it...I could go on, but you should do some research