Canibus has lot of positive effects on the mind, body and soul. It raises your levels of awareness. A few puffs and you feel an instant calm overcome your whole being. The noise level of your mind drops down a notch, or sometimes completely out. Your focus becomes intense. For these reasons it has helped me greatly in meditation. I smoke everyday and have done so for the last 3 yrs. I'm physically, mentally and spiritual at my peak. I'm not saying this is because of the marijuana, but I bring this up to demonstrate, if used properly it is helpful. Marijuana has never had a negative effect on me.
Many people speak of paranoia or having bad trips. What marijuana does is intensify what already is. If your having negative thoughts, those negative thoughts are going to be intensified. Most people are convinced that they are doing something wrong when they smoke marijuana, because it's is illegal. If thoughts of fear are going through the person mind, because they feel some sort of guilt, this fear is going to be intensified and they are going to have a bad high.
The upside is that marijuana intensifys positive feels as well. When you focus on positive feels of peace, joy, love and harmony they take over your being. Now I know people will argue that the feels are "fake" because you are high. Though you may not feel these feelings as intensly once you come off the high, the subtle effects do linger. Overtime of feeling these intense emotions, the will become a part of you, whether high or not.