I think I've commented on this before, but I'll comment again. I smoked weed back in the 70's, but then I grew out of it and got busy with college, work, etc. I've known many people in my life who smoke it on occasion with no intereference to their work or relationships or the segue into hard drug usage. However, for some people it just doesn't work that way and either they start using harder drugs, or they just become unmotivated losers who never have ambition in life; which may be ok, because someone has to work at McDonald's, right? Plus it makes it easier for those of us who AREN'T unmotivated losers to get ahead.
SHould pot be legal? I think so, and controlled like alcohol. Personally, I think that alcohol is WAY more detrimental that pot in every way imagineable, but we all know how the Volstead Act turned out.
AS far as the abuse to your body, yes, any particles that are introduced into your lungs are bad for them. Like Elana said, so I won't repeat it. HOwever, cigarettes are FAR more addictive and because of the copious amounts of particles that are taken into the lungs when a cigarette smoker smokes their pack or two a day, and the detrimental and well-documented effects that smoking has on the human body I belive that cigarettes should be outlawed.
This is a subject that is hotly debated in many different circles and there obviously there is no definitive answer.