I just finished reading this article on chronic marijuana use. They studied 4 patients that smoked 8 grams of marijuana per day. The study found that there was a mild difficulty with attention and concetration as well as a mild pulmonary risk. The study also noted that cancerous tumors have never been found in smokers of only marijuana. In addition, the study suggested that marijuana in conjuction with tabbacco may negate some of tabbacco's negative effects.

Previous studies done on recreational and chronic users were done in Britian (1894), New York (1944), Jamaica (1975), Greece (1977) and Costa Rica (1980). They found no differences between non marijuana smokers and marijuana smokers. In the Jamaican study marijauna users (of 3 spliffs a day for at least 10 years) actually scored higher on intelligence tests that not marijuana users.

All these studies are outlined in more details in this article.
http://www.medicalcannabis.com/PDF/Chronic_Cannabis.pdf