Smoking to any degree of regularity, (anywhere from a couple of times a week onward), has an effect on your short term memory, and energy levels.
You probably already know this, but what many people don't reallise is that it can leave you with a mental sluggishness, even for years after you have quit, that is extremely frustrating if you require high concentration in your life/work.
Imagine that what you see through your eyes, right now, is a film, a movie.
Now, the effect I am describing is equal to losing a few frames per second on the quality of that movie;
For example, when you look from left to right, you lose all that is in-between the exact points that you are targeting, which makes it very hard to concentrait, and commit things to memory.
The fact that you brain is producing an unbalanced ammount of 'happy chemicals' will upset the chemical balance or your brain, and can lead to depression, agin when used regularly.
I'm not even going to take into account the moral reasons why its good or bad, those are for you to agree/dis-agree with yourself, but take it from me;
I come from a long line of hippies, I have known people on most levels of drug taking/drug abuse, and I have indeed been one of them for several years.
The effects of canabis can be subtle, and long term, and I am still finding it difficult to concentrait etc after about a year of quitting.