This is a tricky one. I know that marijuana, in and of itself, is not addictive. What people can become hooked on is the way it relaxes you. Another issue is the potential damage to the lung tissue. Just as with smoking cigarettes, any time you inhale smoke into the lungs it poses a risk of being not only an irritant, but a carcinogen as well. With that said, marijuana smoke has no tar and the average marijuana smoker does not smoke as frequently, or inhale the same volume of smoke, as the average cigarette smoker.
I'll put it like this - I don't think God created marijuana for humans to smoke. I don't believe any plant was intended to be smoked, it just so happened that at some point, ancient civilization realized some plants could be used to induce certain feelings when burned and taken into the lungs. Of course, just because something is used doesn't mean it is sinful. There are many herbs and plants used for medicinal purposes also, herbs that can be applied to the skin to aid burns and plants that can be ingested to quell an upset stomach, etc.
I'll admit, I have smoked marijuana in the past. I don't any more and actually stopped long before becoming a Christian, because I am a mother. Marijuana never altered my thinking abilities. I never felt out of control when stoned. I'd, personally, rather be in a room with a bunch of potheads than a bunch of drunks.
This is one of those conviction issues, I believe. Since this doesn't fall under any Scriptural command I think it would fall under "all things are lawful, but not all things are beneficial" category.