Vaporization's the clear winner in the health department: while burning cannabis plant matter (just like tobacco and any other smoked product) creates a lot of tar and other carcinogens, a proper vaporization releases primarily just the essential oils of the plant because it stays below the point of combustion. These essential oils contain the majority of the cannabinoids and terpenes which give cannabis its effects, so in essence, vaporization gives you all the good stuff and very little of the bad.
Though many studies show that these harmful byproducts of cannabis smoke don’t normally have the same negative effects that they do in tobacco smokers for a variety of reasons, most users (especially those with compromised respiratory or immune systems) would rather play it safe where health is concerned.
However, there may be a dark side to the ostensibly clean vapor produced by portable vaporizers: there tons of these devices on the market, and many of them are made in China with minimal regulation and oversight. It has been argued that some of them contain glue or heavy metals which can be off-gassed at high temperatures and inhaled by the user, although there isn’t a lot of data out on personal pen-style vaporizers in general, as they've only been in existence since 2004.
That said, there are plenty of reputable manufacturers in the marketplace, and the challenge is finding a vaporizer (especially a portable one) made with quality parts. To avoid some common pitfalls, stick to vape pens with ceramic or titanium heating elements which generally are of higher quality than those made of steel, glass, or any kind of fiber wick system.