Have used both and had relationships with both companies. They can help fill a need, particularly if managing across Google and Bing and social. However in my experience, they lagged behind on access to the latest features, and there's an open debate on how effective their bid strategies can really be vs. Google when they cannot make auction-level decisions like Google can. The flip-side to that argument is that for some advertisers, they may not have their conversion data in Google, or may have other bid considerations factoring in that can be better handled with an external bid platform.

Ultimately, this needs to be worth the price though when their fees are measured in % of media.