Hey David!
I manage all of our retargeting and online marketing efforts here at Moz. We actually use AdRoll so we might be a little biased.
With that said, I've had nothing but great experiences with AdRoll. As John mentioned above, in order to run retargeting ads through the Facebook Exchange, you have to go through one of the ad partner networks. So, that's one major advantage right there of running with AdRoll over Google Remarketing.
We've seen amazing results with CTR's of 3-10% on Facebook retargeting ads (especially newsfeed ads) so I highly recommend at least testing it.
I also agree with John that AdRoll's customer service is fantastic. We have a dedicated account team there who is super responsive and all around great folks to partner with.
Hope that helps!
-Justin