I have some experience with what this stuff can do to people (family member had some trouble with it many years back, they have long given up using it though). My view is that although cannabis causes harm, more harm is caused by it being illegal (harm through the drug trade, harm through people committing crimes to feed their habit, harm through people who take cannabis being pushed by the drug dealers into going for the harder and more dangerous drugs) than would be caused by legalizing it, regulating it (where it can be made/sold, how it can be made/sold, where and how it can be consumed, driving whilst under the influence etc) and taxing the hell out of it.
I especially think that the federal government and its agencies need to recognize that the people in various states have decided to legalize the use of cannabis under certain circumstances (some states have legalized it for medical use, others for various kinds of recreational use) and respect the will of the people in those states by ceasing all federal enforcement against people who are carrying out acts legal under state law in these states.
Better yet, eliminate all federal laws (and enforcement) related to cannabis and allow states to control it as they see fit (in those cases where state law isn't up to what it shold be with the states relying on federal law instead, the states can be given enough time to pass new state laws prior to the federal repeal comming into effect)