I am always amazed at the amount of support and outcry I hear on the side of graffiti when it comes up on the internet. Not that it often outweighs the people who say "hands off other people's shit", but the number is still much larger than I expect.
It all depends on context, imo. People should generally leave other people's private property alone. But if it's an ugly cinderblock wall downtown, some kind of drab piece of infrastructure, and the graffiti artist is actually making an effort to make it look good - it can be public art