It's a major turn-off and unattractive.
But I've dated girls who've smoked in the past.
If you really like the person, then it doesn't matter if they smoke. What bothered me more is either:
1. Me really liking them but not allowing my feelings for them to grow because I didn't feel she was serious about quitting, and I didn't like the idea of getting emotionally invested in someone who could develop cancer in the future and die. (seriously)
2. Them viewing me trying to help them quit as me "controlling" them or telling them what to do; instead of trying to help them do what is in their best interest health-wise.
To be honest though, I remember dating my exes who were smokers. At first, it was really unattractive that they smoked. But then, when we would kiss and I would taste the cigarettes, it was kind of cute. It's like, I know it's not the most appealing thing in the world, but it's "HER", the girl I like, so I didn't mind it. After a while, I actually liked it, and thought she looked sexy when she was smoking.
In the beginning it might be unattractive, but as the guy likes you and his feelings for you start to grow, he'll understand that it's just a part of who you are, and you'll love it (but still wish you'd quit; not because he has anything against smoking, but because he wants you to be healthy).