Hmm...
From the ages of 13-17 me and a group of other people used to smoke a fair amount of pot. Every day. At the age of 18(ish), I and a few of my friends stopped, and a few carried on. The ones who stopped generally moved on and are doing much more with their lives (job, happiness, socialising etc), and the ones who didn't stop are still smoking every day, they are doing very little (even though some had HUGE potential), are mostly unhappy and generally spend a lot of time on their own really just getting high and doing little else. Even on the rare occasion when they come to the pub, they always get high beforehand. And we're all 25 now.
From my experience, cannabis is a pretty nasty drug. Obviously, it could all just be coincidental that those who stopped moved on, and those who kept on smoking didnt really change at all. But, that's too much of a coincidence for me. I'd like it to remain illegal.
It obviously comes down to how you use and abuse it, if you just smoke a little every now and again then it's no real problem. Though, I think it's very easy to get in the habit of smoking it multiple times throughout the day though. More so than alcohol.