I agree that Guerrilla Marketing can be controversial. I think its important to take note of laws your city may have in place before you use this type of marketing for your business. Defacing someone else's property in a form of graffiti is, I think unethical, but there are other ways to use Guerrilla Marketing in a way that is legal.

The No B.S. Grassroots Marketing book by Dan Kennedy and Jeff Slutsky has more examples of Guerrilla Marketing and how to do it the right way if you're interested!