I like the new direction they're going with analytic capabilities and dashboards that allow the user to make decisions and not pull data. I also think that they have a respectable platform that scales decently well.

always filled my impressions with a paying ad

Advanced features for the industry. They are already support real time bidding and also can handle being your supply side partner at the same time. They definitely make it hands off for you if you want to use them as a SSP so that can be nice that they can do all the optimizing for you.