You have to remember everything that goes with it. It can disrupt your job (if you have one), your family life, your uni results. Even a huge night on the booze on saturday can fuck you up for teh next week and put you out of sync. Long term weed smoking just disrupts normal life (unless you are a stoner, in which case it IS normal life).
Even if you aren't worried about that shit think about the financial costs of it. It is more expensive that cigarettes and booze to buy, the dealers are shifty and you might get caught and have to pay a fine.
I actually found a pretty cool calculator on the internet when doing some other digging for credit cards which calculates how much you will spend in your life on your biggest vice. Check out how many hundreds of thousands you will spend in your life and see if its worth it
Mozo - Race for Australia's Vice President. I doubt it...