Some people will argue that public graffiti straddles the line between art and vandalism, but I'm more of the "vandalism" opinion than anything else. I can appreciate good graffiti when I see it, and some of the social statement pieces are absolutely fantastic (here's looking at you, Banksy.)
But I also think that some graffiti is simply pointless and only damages the property it's sprayed onto, like when someone tags their name or a phrase onto the side of a building. That to me is just entirely without merit.