Is marijuana really that bad?

I have a couple friends who know other friends or family that smoke weed. I was personally never involved with it, so I don't really understand, and would feel too dumb to ask them in person ^~^ My only viewpoint is that I'm for medical marijuana, but that too much of anything is a bad thing.

Can you guys explain your viewpoint on cannabis? How it may harm you, how it may not harm you, etc? Please provide links to back up your statements if you can ^_^
Yes (explain?)
No (explain?)
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Is marijuana really that bad?
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