I try not to judge people by their habits. I try really hard.
I know several people who smoke. I've dated a few smokers. I've lived with a few smokers.
In the end, I just hate the idea of smoking. I have a huge personal vendetta against cigarettes. Not against smokers, mind you. I do, and have, loved many smokers. But cigarettes - they are murderers.
Luckily my husband doesn't smoke. He never has, so I didn't have to worry about his health until i started nagging. I would nag, too. I'm not above nagging when health is involved.
So... deal breaker? No, not really. You fall in love with a person, not his habits... but the cigarettes would not survive.