I used to be a heavy toker, and do admit that there are downside effects; most especially, an overall "slacker" type attitude towards life, and an avoidance of conflict to the point of ineffectiveness. Again, that is with heavy use. Although I was a heavy user and immersed in the culture, I would get in arguments with my cohorts who believed that it was natural and healthy, and touted its cure-all properties. Knowing that there are vaporized and pharmocological methods of delivery, there is no way that smoking pot can be considered healthy. Introducing any kind of carbon based smoke into one's lungs on a daily basis, via joint, pipe, or bong, is never healthy. My suspicion is that non-smoking lung cancer sufferers didn't get it from "second-hand smoke", but through constant pot smoking.
After quitting it took me several years for the effects to totally go away. I think your on to something with the lung cancer, pot is packed full of carcinogens.