I'm not a big believer in the "Graffiti is art" argument. Most graffiti is just mindless defacement. "Tags", pointless meaningless signatures and "I love Tracey" do not qualify as art. 99.99% of graffiti makes the area in which it is placed look messy, decrepit and run-down.
The work of Banksy however, is worth a second look. Clean, simple, striking and often packed with meaning.