How addictive is marijuana?

Depends: An addiction is a brain disease. It causes a person to engage in behaviors which may lead to dire consequences. The person knows that but engages in the behavior anyway, because they are addicted. It is all about the consequences. Is a person missing work or school or spending too much money or getting in trouble with the law? Cannabis use can lead to addiction, especially with regular use.

Mild, but...: Coffee has a more severe withdrawal syndrome than marijuana, but no one has problems quitting coffee. A really harmful addiction is one in which a person craves the drug (activity, videogame, etc.) and pursues it despite knowing its very high personal cost; 1 person in 10 cannot stop drinking alcohol though it's easy for the rest of us. Stoners who lose their weed supply seldom grow desperate.