I figured you'd want scientific studies as opposed to that site, which on second glance, while fairely balanced, doesn't citate it's sources very well:
Tests of causal linkages between cannabis use and psychotic symptoms.
Relationship between Cannabis Use, Schizotypal Traits, and Cognitive Function in Healthy Subjects
(I particularly like the quote "cannabis use may be a vulnerability indicator for schizophrenia.")
Cannabis and the brain
Cannabis or health

You'll notice not all those links are bad. I'm not trying to scare you or anyone away from smoking up, but until you witness someone go downhill under the blatant strain of smoking way, way too much, then you really don't know the risk without looking it up yourself. No drug is without it's dangers, and it hurts no-one to aknowledge the fact.
:thumbsup: (apologies if my other post sounded a little out of line)