Let me throw my two cents in...
Most of grafittis are crap. However, in certain places and made by true artists, it is nice.
In Québec City, there is a highway going from Beauport straight to the "High Town" of "Old Québec City". That highway is passing right above a poor sector of the City. Well, I don't know if the guys who did that were paid or not, but some artists (believe me, those grafers are artists) painted some wonderful things on the pillars holding the highway up. I found a few pictures on Googles of those artworks. Sorry about the size of the pictures.