I guarantee you that you will not like what these say if you are a die hard smoker, but there are some very interesting facets to these studies.

Effects of cannabis on pulmonary structure, function and symptoms -- Aldington et al. 62 (12): 1058 -- Thorax
Compared to smoking cigarettes.

Caution on this one, federally sponsored, but sounds like a legit study
Respiratory symptoms and lung function in habitual... [Am Rev Respir Dis. 1987] - PubMed result
Compares to non smokers, cig smokers and mj smokers.

Arch Intern Med -- Effects of Marijuana Smoking on Pulmonary Function and Respiratory Complications: A Systematic Review, February 12, 2007, Tetrault et al. 167 (3): 221
a different opinion for sure

Effects of cannabis on lung function: a population... [Eur Respir J. 2010] - PubMed result


[Pneumotorax and cannabis] [Minerva Chir. 2009] - PubMed result
I wish to comment that as a former paramedic, I have treated patients who acquired a spontaneous pnuemothorax from holding the smoke during a cough fit. This can expand what are called 'blebs' and cause a pneumothorax.

The studies vary greatly and have similar findings also. Face it smoking is bad no matter what you smoke. Makes me want to refine my concentrate making.