Its better than alcohol, there hasn't been a cannabis anonymous yet... Seriously though, look at Amsterdam, its legal there. There's coffee shops there that have it on the menu, they try to keep everything in a safe environment and off the streets where bad dealings happen and poor quality skunk weed arises, that's the stuff that'll cause any type of mental damage natural good stuff without chems have a negligible effect on your mental health. There's a concept of "if they want something, they'll get it one way or the other." Might as well tax that *****.
On the other hand though, it has to be closely regulated because it affects each person in a different way. Some people can drive high, some cannot. The legislative office I work at in California tried proposing a bill to pass through to congress that limits the amount of any drug, including weed that you can have in your system while driving and it got totally shot down by every pro weed group and special interests out there which is pretty godd@mn stupid. Also even if it is legalized, depending on your profession, you still can't use it, especially in medical, law enforcement, homeland security and other government agencies.
As for medical effects, I believe its good for certain people and bad for others. I have veteran buddies comming home from Iraq who've been shot and are missing pieces of their stomachs, limbs and some sanity due to PTSD and medical marijuana is a god's blessing to them, helped relieve the pain enough so that they can get their life together and start working and living a normal life. The bad crowd would be teens wasting their life away, smoking for no apparent reason, or to just look cool. Even worse, the d-bags that smoke in public when people and their children are around.