I haven't touched grass in over twenty years but if alcohol is legal, it should be as well. Alcohol is physically and mentally/emotionally addictive but the THC in Marijuana is mostly mentally addictive. I've never read a study that stated anything otherwise. That said, it shouldn't be abused any more than any other drug including alcohol. Personally, I've seen more people addicted to prescription medications such as pain killers or antidepressants than either alcohol or Mary Jane.
Legalize it and even though I'm loathe to the practice, regulate it. When I was younger, one guy I knew bought some of the street that was laced with something like PCP or LSD. Fvcked him up in a major way. Also, as Iamtdg mentioned, there are different varieties with different levels of THC. The last time I smoked some was in '95 and even though I was only a few hundred feet from my apartment, I barely made it back there before falling out on my bed. That was some wicked strong stuff and I just said never again.
At the same time, people should be able to put whatever they want in their body and deal with whatever the consequences are. It's not like the people that make these laws are all physicians.