The combustion of any organic matirial releases tars and carsonagenics, so yes cannabis smoking is bad for you and can give you lung cancer. It also fucks with your short term memory, just speak to someone who has smoked every day for 40 years then the next day ask them what you talked about.

Now none of this has been proved yet as no credable tests have been done on it yet and i guess it is possible that some chemical in it may reverse the effect of the carsonagenics but this seems pretty unlikely.