I think for most people it is a sin but it is not in and of itself a sin (like so many other things.
I am a smoker and have smoked since college. I was raised by parents who were virulently anti-smoking and in a Church where I was taught it was a sin (period). When I started smoking, I was rebelling against a lot of things in my life - some rightly, some wrongly. A large part of my taking up smoking was out of rebellion. Looking back, I view the rebellion as the sin, not the smoking itself. As a matured and learned who I was and became more responsible, I stopped rebelling. I did things that were because I made a conscious choice to do them and not out of rebellion. I did things in line with my beliefs instead of out of rebellion. I of course slipped up and sinned but mostly I would repent and make a true effort to not repeat those sins.
There was one exception - smoking. I was hooked. Since then, I have tried to quit, sometimes I've stopped for a while, but it is a strong addiction. Those on here who say you become a slave are right in most cases. There are a few people out there who can smoke a little and not be addicted, but that is not most people and it certainly is not me.
I hate that I am a slave to it but it is what it is. I do try to stop but it is extremely difficult. I still keep trying, though. There is a lesson for me in all of this and that is to not judge others so harshly for what they are going through and for their sins (or what I perceive as their sins).
As I look back on my life focusing on my health, smoking was not the worst thing for me, believe it or not. I am not a super heavy smoker; I'm not trying to make excuses or anything but in reality I don't smoke very heavily. I've also always smoked lighter cigarettes. Due to this, I have luckily and by the Grace of God not developed any serious smoking-related illnesses; I know I need to quit so I reduce the chance of getting them, but so far I have not (and many people do go through life smoking and don't develop smoking-related illnesses).
Anyway, I certainly am not saying I want to keep smoking but when I said it hasn't been the worst thing for my health I really believe that. Worst than smoking was my poor eating and exercise habits. I was overweight all my adult life till abou 42 years old, 70 to 100 pounds overweight. I also had a sugar addiction. I think these things have hurt me more than the smoking. I fear that future health problems I may have will more be a result of my sugar addiction than my smoking. I am still pre-diabetic although I have drastically changed my diet.
Sugar is a good example to compare to smoking. Sugar and tobacco addiction are both devastating. Are they sins? Not necessarily but often they are or they start out as sins.
But here's the difference: Many will view a smoker as sinning but the same people will not view a person with a sugar addiction as sinning. However, the two are similar addictions and both can be (and often are) sins.
The subject of sin is relative to circumstances in many cases. Is it a sin to drink alcohol? It can be and often is, if it is to get high or if it is in excess such that it harms the body. But if you use alcohol medicinally it is not. Even if you use alcohol moderately it is not a sin, it can even benefit your health when used moderately.
It's not the tobacco, alcohol, sugar, junk food, etc. that we use so much as how we use them and what role we give them in our lives. The sin isn't in those items, it's in our use or abuse of them. A hammer is not a sin. Hammering can be a sin if you're hammering people's heads, but it's not a sin if you hammer a nail; or it could be a sin if you hammer a nail, if you were one of the Romans who hammered a nail into Jesus' hands.
So the best thing to think of here is it's not "smoking" so much that's the sin as what the role of smoking is in one's life and what their heart is in doing that action. I don't beat myself up for it anymore because I have an addiction that I'm fighting. Just like I don't beat myself up for eating sugar anymore because I have beaten that addiction. I don't think a person with a food/sugar addiction is sinning when they overeat if they are truly making an effort to stop. A setback or failure to break the addiction is part of the process. It is the heart of the person where the sin does or doesn't happen.