I was really upset when I saw in the news that some of my hometown's lovely murals had been vandalized. A while back, my town decided to hire art students to paint murals on the walls of some downtown buildings representing various historic aspects of the town. They are beautiful and very well done. But some people went and spray painted some red squiggly lines over them... nothing to be proud of!!! Nothing artistic... just trash.
Normally, I'm all for graffitis when they are well thought out or simply beautiful. I consider graffitis urban design. Similar to what I do except on a large scale and with a different canvas. I really enjoy the kind that incorporates the environment into their art. In montreal, we have a pretty well known artist, Roadsworth, he mainly uses street, parking, crossing lines and creates some fun concepts! It really brightens the city and doesn't hinder anyone.