Don't turn this into a pro pot thread - or even bother advocating pot at all. I mean, yeah it's not that bad in the end but it has its negatives. Decreased motivation, decreased libido, risk of schizophrenia requiring hospitalisation (I work in a hospital, next to the guy in charge of child psychology in the MR department - most kids who are admitted have drug related problems directly associated with smoking weed), a host of neurotoxic effects which can result in decreased STM (short term memory) recall, slowed reaction time - and in addition to this you can apply the gateway drug argument. In regards to the neurotoxic effects - I wrote a 1000 word essay on this for uni if you would like a read. In fact, if you're keen just look on your local uni library database for articles relating to the neurological impacts of delta-9-tetrahydrocannabinol, there are PLENTY.
In regards to the gateway drug argument, I think this is generally pretty accurate (not always, given the right environment). I went on to experiment with LSD and mushrooms (much more intense psychoactives), plus ritalin if that even counts (lol). Lots of my friends went on to the same thing, along with ecstasy and codeine abuse.
Despite all of this I still smoke pot on a regular basis. For me it's harmless fun when I have nothing else to do, and allows me to enjoy music on a whole different level. When I really think about it though, I would probably have just as much fun/be better off without it. It really depends on the situation. If you smoke with good mates semi-regularly and don't let it affect your job/work, then I think it's pretty much harmless. Once you start abusing it and moving onto other drugs, that's when the problem starts. Also, it's sometimes hard to tell when it becomes a problem because the changes can be so gradual/subtle.
Btw nobody should touch this stuff if they have a history of mental illness in the family. BIG no no.