i just smoke pot, but it's definitely been a problem in several of my past relationships. i'm always open and upfront about it, and the girls have claimed at the beginning that they were fine with it. But as time went on and they realized i was serious about how much i liked it, and that i didn't have plans to stop, they started getting progressively more bitchy and passive-aggressive about it. In retrospect, they only tolerated it because they assumed they could change it.
Current gf could care less about it (though rarely smokes herself). It's fucking fantastic.