It goes both ways, I think.

I know people who smoke and get completely mashed and disfunctional. I also know people who are successful and produced some of their best work while high.

I think it really depends on the persons attitude. The people who are successful would have been successful anyway while the people who aren't wouldn't be if they didn't smoke weed. I think weed acts as an amplifier, good and bad. Of course, this doesn't account for the people who are successful in what they do, just not while smoking weed, I guess they exist too.

Also, I think there's a danger that the person smoking may not notice when the positive effects start turning negative. One person I know very well used to work really well while smoking, but recently has become more agitated when not smoking and the weed really just brings her to normal performance, rather than above normal performance like it used to. Note that I noticed this after she had been smoking very regularly for maybe five years, so I possibly long term, the negative effects begin to overshadow the positive, though one person is hardly a scientific observation.