I've been smoking daily for around 3 years now. Whether or not marijuana's usage can develop into a physical addiction, it certainly can become habitual. And if it can be physically addictive, I can only assume that it's relatively minor based on my experience and experiences from those that I know personally.
I'd label it more as a mental addiction, but one that would not normally develop into a debilitating one. Of course, even the internet can be mentally addicting. Nearly anything can be. I became surprisingly bored, restless, and upset when I didn't have the internet installed at my apartment for a week.
Marijuana is sort of like that for me. It's not necessary by any means, but I've grown used to it and quite like it. The same can be said for many non-necessities in my life. When I don't have it for a while, it seems a tad bit harder to focus, it takes me longer to fall asleep, and I might develop a headache. This usually subsides within no more than 2-4 days. Compare that to caffeine, tobacco, alcohol, or even OTC and prescription medications.
For me, that's hardly any cause for concern. Couple that with the fact that it has helped me become a better person over the years, has helped with the slight bit of anxiety I have, helps me fall asleep at night (would normally take 30mins at least before), AND is downright mentally fascinating and fun for me, I see it as much more of a positive aspect of my life than anything else. I try to consume it in non-smoking methods for health reasons.
As for it being potentially dangerous, I don't think children and those with developing brains should use it. A teenager trying it out a few times probably has nothing to worry about. I do think those with pre-existing conditions, especially those related to the heart, should be cautious. I know marijuana can increase one's heart rate and interacts poorly with some medications.
While I do think marijuana is fairly safe, I don't buy that it's impossible to overdose on. I've had occasions where I've had too much. Such situations can actually be quite terrifying and convince the user that they're going to die. I'm not even talking huge quantities...even a large bowl's worth of good material can put you over the edge. While I think the vast majority of people that consume too much will end up being perfectly fine, I could see it causing problems for some (see what I mentioned above).