The few bad things about (non-medicinal) marijuana use are:
-if you smoke everyday, you'll get a hazy, cloudy head feeling and you'll be forgetful.
-(from smoking) you'll be out of breath very quick.
-(only for SOME people) you'll be less included to do something physical (like running?)


That being said, the ideal frequency to smoke if you want to maximize the MANY good effects, while minimizing the negative effects is around 1-2 times per week. This ensures, you wont get the above points. and you'll keep your tolerance down, which is always a great thing. :thumbsup:
-------------
The above is based on my and other people's experience.