My parents did both and nothing good came of it. As soon as my husband and I decided to have kids, we left our vices behind us. It was hard, but it was the right thing to do. Our situation is a bit unique, though. We don't have any nearby family to care for our kids, so we're on call every second of the day. The one or two times a year that I have a beer, or a glass of wine, I feel guilty and worry about a sudden accident happening and not being able to deal with it. I guess that sounds a little paranoid as I write it, but to me, it's just not worth it.
I also burried both my parents early from smoking-related illnesses. I never wish to put my kids in that situation.