I've had friends date my exes, and it really is a case-by-case thing. In the end though, I'm pretty forgiving. Even in the one case where I told my friend that if she went ahead with it the friendship would be over, she went and dated him anyway and I got over it. They dated for more years than he and I did and she remains my best lady.
I don't think I would date a friend's ex if I was single, though. The temptation is not worth the potential drama and friendship heartache.