Let's take a look at Erowid to answer this one.

Good

Potentially cancer-fighting (that's a biggie)

Euphoria

Relaxation

Creative, philosophical thinking

Increased sensual awareness

Increased appreciation for music

Pain relief

Reduced nausea/increased appetite (though some might say that last bit isn't all that good)

Sharpened colors, lights

CEVs

Time dilation

Bad

Headaches

Dizziness

Lightheadedness

Dry mouth (fuck! Wouldn't wanna risk that happening to me!)

Clumsiness

'

Personally, I can attest to most of the positive effects, but the negative effects, not really. Some lightheadedness - not really that bad - and the occasional dry mouth.

Possible risk of mental illness development if you are genetically predisposed. No cancer risk. Risk of memory loss and laziness if smoked heavily (five joints a day apparently).

Versus

Happiness, perception shifts, relaxation, bliss, heightened senses, everything tastes, sounds, and looks awesome, extra creativity, allows for things to be thought out in a different order (difficult to explain, you have to try it).

Basically, weed is fun, and better for you than alcohol or cigarettes, (which are also fun), and if you are predisposed towards mental illness don't smoke weed. If you smoke weed, don't smoke too much. Don't lock people up in a cage for years just for enjoying themselves.

The good does outweigh the bad and this is why marijuana should be legalized.