Us serious artists have to contend with meaningful questions about art such as "Is graffiti art?" I don't have the answer, but my thoughts have been heavily influenced by having the word SUMO permanently etched in the glass of the door to my home. I guess I am not a big fan of the destruction of personal property.
When it comes to the graffiti-ing of advertising in the public space, however, my view is different. I consider advertising visual pollution, filthying up the world for profit (or as They call it, "creating a branded environment.") And I think when people alter billboards and posters, it's hilarious.
I particularly never get tired of the blackening of teeth, especially the teeth of computer-generated pretty people who inhabit artificial worlds. It's like spoiling their perfect party.