It's a deal-breaker for me. Smells bad, dirty, unhealthy. This is much more of a turn-off than being tubby or dull. And I can always tell -- the taste and smell hangs around for ages. It's probably not a rational way to think, but I'm a passionate man ;)
It is a non-issue with me- not a turn off or turn on. I smoke occasionally, usually when I drink, and my boyfriend is a regular smoker. I wish he would smoke a bit less for the sake of his lungs, but otherwise, I don't really care.