Well, here's my worthless input.
MJ has withdrawal symptoms. I know this from first hand experience. While it isn't addictive in the sense that opium and amphetamines are, it still leaves you the day after using with a "hankering" to do it again. And, if you can keep your mind off of it, it isn't very hard to stay away. But, for some people, it is almost impossible to get rid of those hankerings. I have a few friends who simply can't go a day without it, and yeah, I think that's pretty pathetic. But that's them and they're living their life, so be it.
Keep in mind I didn't start using until I was turned 22 (24 now). Sometimes I would go months of using about every other day, then quit. Sometimes I would only do it on weekends for a few months, then quit. Reasons for quitting vary, but usually its because of an employment change and subsequent drug tests.
Now, I can definitely say that after using about every other day, then quitting, I had withdrawal. I was getting night sweats, I was irritable, and had trouble falling asleep. After about a week, these problems went away. However, using once every five days or so, I never really experienced withdrawal problems.
I think a lot of the people who have issues quitting are people who have used since teenage years. I do believe it potentially slows development somehow if you begin using while young.
I have never allowed MJ to affect my life and take control of it. I am a very professional person who pays bills on time and holds steady employment, but, most importantly, I am dependable.
Again, this is just my worthless input. I know a lot of the DARE brainwashed members are in the thread, and to be honest, I respect and understand your opinion because I believed all of the drivel that was fed to me up until my 20s when I realized it simply isn't that big of a deal. Sure it is illegal, and that is a major factor why people don't use. MJ can ruin a life, but, only if they let it.