Yes, Marijuana has been proven time and again to be relatively harmless. I agree with reallifesituation, moderation is key.
Thus far the greatest health risk is smoking related illness, but not lung cancer. Promising research would indicate that cannabanoinds in marijuana inhibit cancer cells from growing.
Lately there has been a new tactic relating to the potency of weed today compared to a few decades ago. This is a false danger on two counts.
1) Actually the THC percentage by dry weight of todays marijuana is only about 2%-4% higher at most than samples tested in the '70's, and a few samples were actually much higher back then. So the potency has increased some, but not like it is being portrayed. A lot has to do with the make-up of the samples as well. 30 years ago you got weed that was unmanicured and contained a lot of weaker leaf matter. Today it is meticulously manicured and contains primarily the more potent buds exclusively, thereby creating a sample with an overall greater THC content than one that contains a lot of leaf material.
2) Marijuana users self regulate consumption much better than say alcohol drinkers. It's effects are felt within a minute and peak within fifteen minutes at most. Alcohol effects are also felt relatively quickly, but peak effects are slower so the drinker's consumption can easily out pace the effects. We have all seen this or experienced this. People don't usually overdo it with pot, and if they do death isn't a real concern as it is with America's drug of choice, alcohol.
Also the higher potency actually results in less being smoked thereby greatly reducing health risks associated with smoking.
If I had to smoke a entire joint in 1975 to achieve the desired effects, today I can often get the desired effect with 2-3 hits from a bong, which also filters some of the tar out as well.
This seems to be a point that is conveniently not mentioned by the anti drug supporters citing increased potency.