Futurama had an episode that submitted in the year 3000 advertisers will be able to send signals in the air that put commercials into people's dreams. Funny that Ralph Nader mentions sleep as the last place you can go to escape ads. In today's world, they're pervasive. It's a good topic for an amusing and entertaining filmmaker (Morgan Spurlock, who previously had success with his anti-fast food pic Super Size Me). Much as with Super Size Me, Spurlock pulls a stunt; this documentary is funded by product placement.
There are certain limitations inherent in such a project. Make a film against product placement, and the worst offenders will not want to help it. In the end he mostly finds businesses I've never heard of (Ban, Sheetz). So we don't see how the bigger corporations go about product placement. That said, we see a bit of how it works, as Spurlock's sponsors send him contracts making various demands.
At times it seems this movie is more about itself than product placement generally, but we do have some good discussions spread throughout the film. It is true blatant advertising is insulting, yet the film poses the question of whether subtle advertising is more dangerous. The presentation is funny, including with the Mane 'n Tail material. (Why didn't that company pay for the publicity? They even got a Wikipedia page because of this movie). The film has some good music and I can testify that it looks great on Blu-ray. The Greatest Movie Ever Sold may not be the greatest documentary, but it's worth a look.