http://www.youtube.com/watch?v=gwDRBm-qbQI
I used smoke myself but ended up getting paranoid as fuck. Thought I was becoming schizophrenic. Though even with that I'd still say legalise it. I decided to quit and had no trouble quitting as even any drugs counsellor will tell you it's not a physically addictive substance.
Most of the reasons people would use to keep it criminalised are in direct correlation to it being illegal. Gateway drug, drug money funding gangs and violence due to said money all due to where we get the drug. I would say that maybe it should have an older age limit then 18 though.
At the end of the day though the main point is we're all grown adults here and should have the option to smoke or not smoke ourselves if there's no across the board reason to ban it