I remember trying to paint one and being completely confused by it. It just looked like little uncoordinated dots to me. Maybe if I'd been as enthusiastic as the young lady in the ad....
She seems really excited, like maybe this is the first time she succeeded in coloring inside the lines? Too bad the 'artist' herself wasn't rendered as a paint-by-number, but I guess that would have been a little too meta for the time.
I got something like that for a birthday one year. I never finished it, but I remember it was a collie dog's head.
I don't think I ever tried paint-by-number, but I remember color-by-number, with colored pencils. Definitely the closest I ever came to something approximating art!
Yeah, well, forget you, Color Master! I color how I want! (Paints all of Paris red.)