http://www.consumerreports.org/health/marijuana-good-for-you-or-dangerous/
tl;dr -
The Good - For chronic pain & nausea from chemo. Helps with MS.
The Bad - there's "substantial evidence between mj use and the development of schizophrenia", low birth weight if pregnant women are users and the eariler someone starts using, the more apt they are to "problem use".
There's more, but read it at your leisure.