>Smoking marijuana does not lead to addiction.
It seems the general consensus (though I've seen a few articles that may suggest otherwise) is that it doesn't lead to physiological dependence. But not everyone agrees that addiction is defined by physiological dependence.

As i stated before, the MRJ has found that marijuana causes a long term increase in IQ relative to the genral population. The study did not however test specific cognitive domains such as learning, memory, psychomotor coordination, or the ability to weave baskets underwater using only your toes. The FACT is that it increases your global IQ score.
Are you talking about this article?
If so, that's an exaggeration. They actually said they found no difference in change in IQ scores between non-users, former users, and regular users who smoke less than 5 joints/wk. They still found a negative trend for users who smoked 5 or more joints/wk. And anyways, the authors warn that their results (both for the positive and negative trends) should be interpreted cautiously because of the small number of subjects studied and the relatively short duration of the study.

Water pipes will reduce "tar" levels significantly both because major components of "tar" are water soluble and through mechanical action. THC is not significantly water soluble and consists of generally smaller particulates, reducing the effect of mechanical filtering in water pipes.
I mis-wrote. I meant that it said it increases the tar/cannabinoid ratio, even though it does decrease the amount of tar overall.