Cannabis does have known negative health effects. It causes problems with short-term memory formation, impairs motor coordination, reaction times and judgemental facilities, can worsen(but not cause) schizophrenia and it's technically toxic, although you'd have to be injecting pure cannabis directly into your bloodstream to even approach a lethal dose. It doesn't cause addiction like nicotine, morphine or various other drugs, as it does not cause any withdrawal symptoms, but the feeling of well-being it produces can be somewhat addictive. Overall, it's not even as bad as tobacco or alcohol.
I've heard arguments that weed is a gateway drug. Certainly, almost all hard-drug addicts have used cannabis before in their lives, but I don't think it's quite that simple. Since cannabis is illegal, it means you have to buy it from drug-dealers. And since dealing in cannabis is already illegal, most of those deal in harder stuff as well. It only makes sense from a business perspective for the dealer to try and persuade the cannabis buyer to try harder, "better" drugs since those come with an increased profit margin. You see what I'm getting at? Legalizing cannabis would, in my opinion, actually decrease the usage of other drugs. Furthermore, it would deprive organized crim of a big source of money. So I'm all for legalizing the stuff.