The plumber's ad is so ubiquitous that, if you know Chinese, you may have the impression that Hong Kong has tons of blocked drains. Does this make the plumber's ad famous?
Nope. It is because of one person, Stephen Chow.
An actor, screenwriter, film director and producer, Stephen has produced numerous blockbuster comedy movies which won him several movie awards and grossed great proceeds in Hong Kong, China and from most Chinese speaking communities. It is he who gave birth to a special genre of acting called "mo lei tou" (or silly-billy). The comparable English equivalent may be Austin Powers.
So in one of his movies, a scene showing the plumber's ad somehow arouse the interest of the audience in China. With millions of millions of population, the plumber's ad attracted much attention and lots of discussions.
These graffiti kind of ads proliferate in old districts like Sheung Wan, Yau Ma Tei and Kwun Tong. Take the MTR train to there.