very few scientific studies have actually been published that elucidate the long term effects of exclusive marijuana smoking on the lungs. Of what little legitimate research has been done, it has shown that it doesn't cause tar buildup (the assumed catalyst for tobacco related cancer), but makes the inside of your lungs lose their cilia (little hairs) and makes them more like your outer skin.

Other than that, neurological effects range from the non-detrimental (larger blood vessels in your brain) to the downright enjoyable (permanent relaxation caused by synchronized alpha brain waves in the frontal lobes).

You shouldn't trust any information about the effects of smoking on the internet, because, surprisingly often, they just state urban legends as fact (low sperm count=myth). If you have resources available, like a university or decent city library, you should research these things from sources that are proven to be non-biased and purely scientifically based.