Various states have legalized marijuana for medical use for years, and Washington State and Colorado just legalized marijuana for recreational use. My thoughts:
1) Marijuana is known as a “soft” drug, but in fact it is extremely harmful. Most users experience paranoia or anxiety, which can often persist after the drug use is discontinued (so much for the users’ goal of being happy and mellow). Most alarmingly, recent, well-done epidemiological studies have found that teenage pot use increases the likelihood that the person will come down with schitzoprhenia or psychosis (that is, major psychiatric illnesses that basically can’t be cured and ruin your life)! As R’ Moshe Feinstein mentioned in a teshuvah forbidding marijuana, it can also have other negative effects, such as lack of motivation and mental dullness. We should stay far away from this drug and insist our children and their friends and our friends do too!
2) That said, it should be legal! America has the world’s highest incarceration rate by far, many times the rate of any comparable country, and much of that is because of harsh, often discriminatory enforcement of the drug laws. Incarceration isn’t even a concept in (traditional) Torah criminal law. Nor is banning certain substances. Making it a criminal matter creates more problems than it solves (incarceration ruining people’s lives, violent gangs as with Prohibition, etc.).
3) As a last resort it may be useful as a medicine for some people, so it should be available as a medicine even if it’s not legalized for recreational use, but it shouldn’t be widely used because of the side effects.