My first three LTRs were with people with substance abuse issues. I entered them listening to my heart only. My heart found "bad boys" so attractive. Now, years later, they make me want to hurl....mostly because it's almost always an act, or a sign of extreme irresponsibility or childishness, none of which I respect nowadays.
My current LTR was entered into with about 50/50 heart and head. I was, at that point (8 years ago) finally able to say: is he an addict? is he self-centered? is he capable of a good partnership? does he take care of his own side of the street? etc etc
This doesn't guarantee happily ever after, but it's worked out much better for me so far. The bad boys I meet make me think "Thank god you're somebody else's problem now."