If you're asking if its physically addicting, then no it is not. Your body does not become dependent on THC (the primary chemical in Cannabis that makes you high) and does not go through a withdrawal phase like it does after extensive intake of substances that contain Nicotine, Alcohol, or Caffeine.
But it can be addicting on the psychological level, just like pretty much anything else can be. It's all in the head and does not stem from any of the properties of the plant itself. To blame Cannabis for ruining peoples lives is simply trying to take the focus away from our own shortcomings and weaknesses. It's the same as blaming video games for youth violence. Some people just aren't meant for it, it's what makes us individuals.
For those who claim that it makes you 'lazy' or 'slow' have to keep in mind that there is more then one strain of Cannabis, each one bringing its own unique characteristic to the high. Each of those strains usually falls into one of two categories: Indica and Sativa. One gives what people call a 'body high', which can be attributed to the laziness and slowing of pace that some describe. The other gives you a 'mind high', which stimulates anything associated with the functions of your brain, particularly the 5 senses. This one will make you very active.
As for Cannabis making you stupid, I'm not 100% sure on that although I have read some studies that described that it can have negative effects on developing brains, but so can spending your days devoted to the TV. Keeping your mind sharp is your responsibility, go read a book or anything else that keeps your mind active, especially the logical and decision making regions of your brain.
Oh and yes it will mess with your short term memory making abilities, but I'm fairly sure its easily reversible, just stop smoking if you think its becoming an issue. Your body won't hold you back, in the end, it's your decision to make.