I've found it mostly useless, sure I've visited these websites I'm getting ads for everywhere but I already know the company exists and I'm basically just getting a "we exist!" type ad.
An example would be hipmunk, I think they would probably be much better placed try to use banners that offer deals through retargeting. I already know they exist and have been to the site, the ad that I now see because of that doesn't give me any further information.