Theres a lot of concern about the risk of cancer when smoking it. So don't smoke it, eat it! Weed pizza is a favourite.
As for the therapuetic values, it has recently been discovered that it acts as an SSRI and inhibits the reuptake of Seratonin, which is what some antidepressants, such as Prozac, do. The side affects of Prozac and other SSRIs are a lot greater than with cannabis, especially when it's eaten rather than smoked. It can also be 'vaporised' which heats the cannabis to a very high temperature and destroys the carcinogens but leaves the active chemicals intact or smoked through a water pipe/bong, in which a lot of the carcinogens are absorbed by the water, both of which reduce the risk of cancer.
As for 1 spliff being equivilant to 16 cigarettes, it's a lot closer to 3, due to spliffs not having filters and if your smoking weed your a lot more likely hold the smoke in your lungs for longer. Also, you tend to smoke a lot less spliffs than you would cigarettes so the risk of contracting cancer if you only smoke spliffs is still lower than if you only smoke cigarettes.
I smoke cannabis regularly and I find that my mood in general is much better and I have a lot lower stress levels. I much prefer cannabis to alcohol as I'm a lot more relaxed and in control after smoking than I am after drinking. There have been occasions where I have noticed some side affects, but only minor ones, such as feeling a bit lethargic the morning after smoking a lot or, very rarely, getting a slightly increased feeling of paranoia. In combination with alcohol it tends to make me feel sick, so I'm very careful to moderate what I'm doing if I'm doing both, which I don't very often.
As far as cannabis being a 'gateway' drug, I have mixed opinions about this. I think in general that if someone is prepared to try banned 'drugs' such as cannabis they are more likely to be prepared to try stronger drugs. This isn't anything to do with the effects of cannabis, or any feeling of needing something stronger after smoking cannabis, in my opinion. I think the decriminalisation of cannabis would actually help remove the perception of it as a gateway drug, as it would be no longer banned substance and wouldn't produce the same peer pressure effect to get people to try it.
If I had my way I'd decriminalise cannabis and have alcohol removed from human conciousness. Just banning it wouldn't have the desired effect and would cause more problems. The reason I'd like to be rid of it is many a good night out has been spoiled by drunken people getting agressive and looking for fights, something which wouldn't be an issue if the people had been smoking cannabis instead, as anyone who has smoked it will know it gives you a relaxed, non-agressive feeling of well being, in my experience at least.
If this changes even one persons opinion of cannabis I'll be happy. I've spoken to quite a few people about cannabis and most, not all, people I've spoken to who are against it haven't looked into it and are just repeating media/government/education message "Drugs are bad". After talking to them about it, most people feel undecided, some people want to try it, some people feel the same. I urge anyone who doesn't know much about it to look into it, not necessarily try it for themselves, although it would improve understanding of it, before forming a strong opinion and speaking adamantly on the subject.
Hope you enjoyed the essay
