I personally dont use it, just because its illegal, and it part of my religious beliefs to follow the law as much as possible. However, as far as I can tell, its not much worse for you than alcohol. Its certainly not worse than smoking tobacco, because its generally not addictive. Whenever you talk about smoking anything, you have risks of mouth and lung cancer, which is not cool.
One thing people often say about it is that its a "gateway drug" meaning people start with pot and then try harder things (coke, meth, x, etc.) While its true people often try pot first, most people I know only smoke pot and do nothing harder. In the US, there has been a lot of advertising by the tobacco companies to keep pot illegal (so people smoke cigarettes.) At the same time, it has a long history of asocciation with a lifestyle thats not law-abiding either.
If it was legal, I would probably use it occasionally, like I currently do a hookah (1 or 2 times a year.) There are health risks, but its also relaxing and fun.

It's a plant.  Just like tobacco.  Our gov't subsidizes one and bans the other.  Wonder why?