FYI: The AdRoll folks are indeed great. We've interviewed a handful of their customers as well as customers from many other retargeting platforms and have heard great things.
We see retargeting as a huge space with plenty of room for several players.
DoubleClick is very much the largest and awesomest exchange. But there's A LOT of display ads on the web.
The data we have suggests that DoubleClick offers up about 4b daily impressions to retarget against. The overall retargeting ecosphere that folks like us and a few others are retargeting against is in the 10-14b impressions/day range. DoubleClick's massive and have a lot of the best inventory, but it's still just a part of the whole.
NOTE: I'm sure there's other display-heads on HN who might have different data. Either way, Google's exchange is no more than 30-50% of the total exchange inventory out there.
My understanding of Google's program is that it runs your ad across its DoubleClick exchange. We run our clients ads across DoubleClick just like them... as well as the rest of the ad networks... plus our publisher partner inventory. Google's DoubleClick inventory is a part of our overall inventory.
I'm part of a stealth startup that's handling retargeting for about 30 startups now and I can confirm that it is indeed a brutally effective method for acquiring users. We're seeing that on average, retargeted ads perform about 350% better than any other kind of targeted ad. It seems a little crazy until you recalibrate your understanding of what's happening. Instead of trying to push new information on a user (which no one likes), with retargeting, you're _pulling_ people back to something they've already expressed an interest in. It works marvelously.
Example: We're running retargeting for an SaaS startup that gets a lot of airtime on HN, and they're snagging new users at less than $2/head. Once these folks get on board, they spend on average $40/year on the service. Our little team had a "f___ yeah" moment after the 4th day or so when we saw how well it was working. We're now hustling to hire folks to meet the demand coming from word of mouth.
If any HNers on here want to ad retargeting to their startup, web app, side project, mobile app landing page, or whatever the heck you're building, hit me up at the e-mail address in my profile.
We're still building out our actual product so startups and technical people make for the best customers. Less explaining, more appreciation for results, more understanding when we goof up.