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.
How much wider reach do you get from going to multiple exchanges? It's maybe time I did some research on moving away from boomerang!
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.