i got this from about.com
An analysis of research studies with long-term, recreational users of marijuana has failed to reveal a substantial, systematic effect on the neurocognitive functioning of users. According to researchers at the University of California, San Diego (UCSD) School of Medicine, the only deleterious side effect found was a minimal malfunction in the domains of learning and forgetting.
The studies included 704 long-term cannabis users and 484 non-users.
"Surprisingly, we saw very little evidence of deleterious effects. The only exception was a very small effect in learning new information,"
Grant added that the minimal side effects seen "raised the question of practical significance. If we barely find this tiny effect in long-term heavy users of cannabis, then we are unlikely to see deleterious side effects in individuals who receive cannabis for a short time in a medical setting."
and this from NORML.org
Enforcing marijuana prohibition costs taxpayers an estimated $10 billion annually and results in the arrest of more than 829,000 individuals per year -- far more than the total number of arrestees for all violent crimes combined, including murder, rape, robbery and aggravated assault.
Of those charged with marijuana violations, approximately 89 percent, 738,915 Americans were charged with possession only. The remaining 90,710 individuals were charged with "sale/manufacture," a category that includes all cultivation offenses, even those where the marijuana was being grown for personal or medical use. In past years, roughly 30 percent of those arrested were age 19 or younger.