It is like anything. Too much of anything is bad. Moderation is the key. Especailly if you are in a relationship.
One thing it can do is to desensitize you to sex. I mean, if you already got off looking at porn, you may not be in the mood when it comes to your partner.
Since porn is fantasy, fantasy is always better than reality. Thats what makes them fantasies.
It is an addiction when you no longer want to interact with reality and spend so much time on it, that it effects your life negativley. When you use it as a substitute for social interaction, then it is bad. But you could say that about anything, not just porn.