Weed is a weird drug to classify, which is one of the main reasons its federally illegal (FDA). It's actually a mild stimulant, depressant, and psychoactive hallucinagen all rolled up in one. It doesn't cause visual hallucinations (repeatably, there is sometimes no accounting for a potheads imagination), but it can cause one to have a certain sense of well being that really shouldn't be there in todays farked up society. Some people shouldn't smoke weed, but most people who have something against it without ever having tried it should definitely smoke a joint.
As far as people saying it is addictive, here's the skinny: Marijuana is *not* physically addictive, at all. Marijuana *can* be the focus of a *pre-existing* addictive personality. To say that it is physically or psychologically addictive without qualification is not scientific.
As far as the carcinogens contained in weed: The main carcinogen (cancer-causing chemical) in marijuana is a tar called benzopyrene. It is the main carcinogenic tar in both tobacco and marijuana, and the concentration is about three to six times higher in marijuana than tobacco. That being said, one must consider the total amount used in order to determine a statistically significant theory regarding the intake of the carcinogens. One cigarette contains about 1.5 grams of tobacco. 1.5 grams of good greenbud will get 6 hardcore stoners high for 3-4 hours 3 times (if you roll 0.5g of weed in a blunt). Thats one hardcore stoner high for 54 - 72 hours. Thats 2-3 days. One regular tobacco smoker smokes between 10-30 cigarettes containing 1.5 grams per day. All of that being said, benzopyrene is probably the most dangerous compound found in marijuana, and definitely the one with the highest volume. Meanwhile tobacco cigarettes contain literally hundreds of other horrifically more dangerous compounds than marijuana. I am definitely not saying that the carcinogens in weed or cigarettes are good for you, but even some tobacco smokers live to be 100 years old. Saying that weed is bad for you because it contains carcinogens and likening it to cigarettes just doesn't add up.
There has never been an actual published scientific paper that says that smoking marijuana has any long term negative side effects*edit: other than the carcinogenic effects of the smoke*. Several papers were published in pseudo-scientific magazines in the mid 80's to the mid 90's which claim all sorts of negative effects of marijuana (seriously, from brain damage to birth defects), but these papers can usually be traced back to a series of laboratory procedures approved by the reagan and bush cabinets which were funded by the federal government under one of the war on drugs initiatives.
LSD, psilocybe mushrooms, and peyote are all relatively harmless in a physical sense, but use for some people has caused long term mental problems. These problems are usually linked with pre-existing psychological problems, but at all times should be handled with education and extreme caution.
Pretty much all other drugs are highly toxic and cause cell death in the brain*edit: as well as serious physical and psychological dependancy*. Stay away.