http://www.maps.org/mmj/vaporizer.html

Read that, if your too lazy here are some key parts. BTW these were pulled from studies done by The Multidisciplinary Association for Psychedelic Studies (MAPS) and The National Organization for the Reform of Marijuana Laws (NORML).

"We learned early on that waterpipes don't help filter out undesirable particulate matter, although waterpipes may help reduce certain water-soluble gases." ...

..."Contrary to popular impression, waterpipes don't necessarily protect smokers from harmful tars in marijuana smoke, according to a new study sponsored by MAPS and California NORML (National Organization for the Reform of Marijuana Laws). The reason is that waterpipes filter out more psychoactive THC than they do other tars, thereby requiring users to smoke more to reach their desired effect. The study does not rule out the possibility that waterpipes could have other benefits, such as filtering out gases, but it suggests that other methods, such as the use of high potency marijuana, vaporizers, or oral ingestion are needed to avoid harmful toxins in marijuana smoke."

I'll agree I've never gotten ash in my mouth, and I would MUCH rather take a hit from a bong than a pipe, but science doesn't lie man. Talking about airborne nasties, not just heat, and the once in a while ash (which if you are skilled won't happen, thats not a bust I'm just saying...)

I mean no disrespect, I'm just trying to keep people informed.