Porn addiction definitely exists. Something can be psychologically addicting rather than physically addicting.
Personally, I think that porn is overall a net negative for society, especially now that the Internet is here. Back in the day when "looking at porn" meant you had a few magazines stashed in the closet or you had a few VHS tapes, that was one thing. But now that it's so easily accessible in such vast quantities, it's much easier for the shit to get out of hand.
Then on top of that, it's not just that porn is out there, it's also the KIND of porn that's out there. You can basically find material for any perversion and many people have found that their drive for porn got weirder and more bizarre as time went on, which ultimately had a negative psychological effect on them.