Google informs users that "any effect depends on whether a website responds to the request, and how the request is interpreted. For example, some websites may respond to this request by showing you ads that aren't based on other websites you've visited. Many websites will still collect and use your browsing data - for example to improve security, to provide content, services, ads and recommendations on their websites, and to generate reporting statistics." In fact, "most web services, including Google's, do not alter their behavior or change their services upon receiving Do Not Track requests".
Chrome is actually the last major browser that adds support for "Do Not Track", a feature that is already available in Firefox, Opera, Safari and Internet Explorer. Microsoft enabled "Do Not Track" by default in IE 10 and many sites will ignore the request because users didn't explicitly enable it.
While the "Do Not Track" HTTP header was supposed to make things easier for privacy-conscious users, it's now a useless feature. The word "tracking" is vague, advertising companies are not required to honor "Do Not Track" requests and IE 10 offers new incentives to ignore this feature.
For now, extensions like "Keep My Opt-outs" or sites like aboutads.info are more effective. You can also disable third-party cookies or install ad blocking software.