Almost immediately the violent crimes associated with illegal distribution would cease, the price would reduce so that idiots who take them wouldn't have to steal to get them. Basically look at the improvements one can associate with the end of the First Prohibition and you will see the same benefits associated with ending the Second Prohibition.
It would be far more difficult for the underage to get them as they would sell behind counters where people check IDs, more supportable DUI laws and research into effects could be done (idiots wouldn't be able to claim that MJ has no associated cancers, well unless it really doesn't, for instance because real medical research could be conducted.
You could write whole books about the subject (many have been written about the ending of the First Prohibition).