Coalition does our online paid advertising. So far, they're handling Adwords, Facebook advertising, Microsoft Ads and they are also helping us with Amazon ads (Which I had no experience in before or even knew about).
They have a high management cost (comparatively) which varies on your spend amount with them. But so far that has been a non factor with the amount that we saved. Our spend rate savings (on average) is about 35% over the first six months we have worked with them (which is why we're adding in some more advertising networks). Fortunately, that savings has not resulted in any fall off with sales.
If we can keep the mean and lean spend going, and find some new advertising opportunities, these guys are golden!