You'll have more control as a user. The "why am I seeing this?" link with each ad will show just who was responsible for the targeting information an whether or not your contact info was involved (say, an email subscription). If you object to a company using those methods, you can block their ads.

The system isn't perfect, as it requires advertisers to tell the truth about where they got their info. It's entirely possible that a company will simply lie about the origins in order to hawk their wares to customers. This at least creates a record of that deception, though, and may open the eyes of companies that didn't realize they should get your explicit say-so.