Alcohol can be described as mind altering and intoxicating as well, but not many people have a problem with that. Assuming it is legal, which where I live it is not, I do not believe it to be a sin at all. As with alcohol, if used in moderation and without addiction I don't think that it is fair to classify it as a sin just because it's used less openly than alcohol.
There are varying degrees of getting 'high' and as with alcohol, the more you smoke the less inhibitions you have so if it is used to a point where you are still fully in control of yourself I don't see the problem. And yes, this is from someone who knows what it feels like and I have never felt as if I was sinning against my creator, I have probably done it about 6 times in a 2 year period so it's not an addiction clouding my judgement either.