I've never really considered Buffy/Faith. I think after Faith came back in season 7 it would have worked better. Before that they were too busy being crazy-pissed at each other. ;P

That's kinda why I ship them :D. I think if you can hate someone that much, it's only because you loved them first. But I do agree that it wasn't really an option (beyond hate!sex) before season 7. And if you've read the comics, you know that it's not an impossibility:).