As a person who has smoked enough pot to know what I'm talking about, but who has left it long behind, I feel I should also put my two cents in.
Marijuana is like any other drug, including alcohol and nicotine; no more, no less. It is not evil and it is not harmless, no matter what detractors and supporters may say. It does affect your judgement and actions - or else why would you smoke it in the first place?
It is a stronger drug than nicotine and therefore should be used in moderation and be subject to the same controls as alcohol. THC has slightly less direct effect on things such as reaction time than alcohol does, but it has similar effects on inhibitions and it does affect the quality of your reactions, and does eventually affect the speed as well. Milefile mentioned to events where he was able to function "as if he'd never smoked" when confronted with stress. I will tell you from experience that adrenaline will sober a person up just as quickly from a similar quantity of alcohol or marijuana. Confronted with the same bad news or danger from police, a drinker would function just as well as a smoker assuming they had only lightly consumed, as Milefile had.
AO touched on the subject of marijuana being a 'stepping stone' drug. Of course it is - to a person with an addictive personality. But so is beer a stepping stone to harder liquors to the same type of person. In both cases, you get used to the taste and the effect, and if you are the wrong type of person, your need will drive you towards more consumption and stronger doses/drugs. If you're not that type of person, it won't. I knew a person with a prefectly managed and controlled cocaine habit, too. One person. Unfortunately too many people are the wrong type and both marijuana and alcohol lead to harder use and worse impairment/addiction.
It's not a given that anyone who smokes pot will turn into a futureless dropout. It is no coincidence that many long-term and heavy users of either pot or alcohol are undeveloped both emotionally and intellectually, and are often chronically underemployed. Drinking has exactly the same potential. But it is a given that a person at risk for that kind of behaviour - marginal students, marginal workers - cannot afford to waste the time, effort, and money of having a drug habit. If you don't pay attention at school and don't do your work, then escaping via pot or booze is going to make a bad situation much worse. The need/desire for the next high is a huge, self-made distraction, in a world full of distractions that you can't control.
You can't afford to do that to yourself. Look at dredknot's signature - he complains about not having money, not having a girlfriend - but he's putting even more obstructions and hurdles between himself and those goals. Obviously it bothers him or he wouldn't complain about the situation, but he's not willing to remove the obstructions he can control and concentrate on overcoming those he can't. There are happy winos and happy stoners who don't want any more out of life. But their lives are pretty thin when you look at them.
Why did I quit smoking pot? I got bored with it. I don't drink every night (or even every weekend), either. Simple as that; it never really interefered with my school or work. In my case I realized I was deliberately wasting time and energy getting high or drunk, and I didn't think the benefits were worth it. In fact, beyond the filling of those hours themselves, there were no benefits. All that time and effort was better spent reading, learning, playing my guitar, working on my car, playing games, anything - anything that added to my ability and my knowledge in exchange for the hours and days spent doing it. No matter much fun they may be, alcohol and drugs will never give you that benefit.
A little relaxer once in a while is fine, but habits are habits and as soon as satisfying the habit becomes an end in itself - a goal of its own - it becomes a problem, whether you choose to admit it or not.
