Vaporizing marijuana releases the medicinally active cannabinoids without burning the plant releasing toxic byproducts of combustion that can be respiratory irritants. Basically there are many pros and no significant cons, though some people (usually young smokers) don’t find it as satisfying at first. By Dr. Randy S. Baker (healthtap.com/#topics/vaporizing-vs-smoking-weed).
The following by California Normal, http://www.canorml.org/health/vaporizers:
Vaporization is a technique for avoiding irritating respiratory toxins in marijuana smoke by heating cannabis to a temperature where the psychoactive ingredients evaporate without causing combustion.
Laboratory studies by California NORML and MAPS have found that vaporizers can efficiently deliver cannabinoids while eliminating or drastically reducing other smoke toxins.
Like tobacco, marijuana smoke contains toxins that are known to be hazardous to the respiratory system. Among them are the highly carcinogenic polynuclear aromatic hydrocarbons, a prime suspect in cigarette-related cancers. These toxins are essentially a byproduct of combustion, separate from the pharmaceutically active components of marijuana, known as cannabinoids, which include THC. Although there is no proof that marijuana smoking causes cancer, chronic pot smokers have been shown to suffer an elevated risk of bronchitis and respiratory infections. Respiratory disease due to smoking may therefore rightly be regarded as the primary physiological hazard of marijuana.
Cannabis vaporizers are designed to let users inhale active cannabinoids while avoiding harmful smoke toxins. They do so by heating cannabis to a temperature of 356° – 392° F (error! 451° is the actual point of combustion), just below the point of combustion where smoke is produced. At this point, THC and other medically active cannabinoids are emitted with little or none of the carcinogenic tars and noxious gases found in smoke. Many medical marijuana patients who find smoked marijuana highly irritating report effective relief inhaling through vaporizers. Users who are concerned about the respiratory hazards of smoking are strongly advised to use vaporizers. Alternative devices, such as waterpipes, have been shown to be ineffective at reducing the tars in marijuana smoke.