For me it is a waste of money, but others had success with it.
Below is a snippet from Google on remarketing:
What It Does
Remarketing lets you show ads to users who've previously visited your website as they browse the Web.
When you use remarketing, you'll tag pages of your site that correspond to certain categories you want to promote. For example, you could add a “TV” tag on all of the pages where you sell televisions. You can then create an AdWords campaign to show highly relevant messages (such as ads displaying a special offer on TVs) to people who've visted these pages as they browse sites across the Google Display Network. Why You'd Use It
Remarketing is a powerful way to stay engaged with your target audience. Presenting them with highly relevant ads and offers across the Web -- and making sure your brand is top of mind when they’re ready to buy –- can radically improve ROI. Achieve even higher conversions by combining other targeting methods, such as interest categories, demographics, or keyword campaigns. For example, if you wanted to expand the reach of your fashion brand, you could target only women between the ages of 18-24 who have not yet visited your site, and present them with an offer that is tailored to them.
I've heard that some webmasters visit some sites doing heavy remarketing at once until they end in the last checkout page of every one of it. Then, they just back to their own websites and browse some dozens of pages seeing how the fight between remarketed ads rises the CPM of the site. Brilliant? A waste of time?