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I am new to them as well but if you tell them what category of ads you think would best perform they will pass it on to the Media.net developers and after 2 weeks you will start to see those ads appearing. I am only running them on certain sections. I might check the visitorís country to determine AdSense ad or Media.net ad. Not sure yet but now the ads they are displaying now are definitely relevant. I might just give them the whole site for 2 weeks and see how they compare to AdSense.
I've been using media.net for over a year. It took some time for the ads to target well, but once they did, they started to perform well. RPMs overall equal adsense. It seems to do best when it is not the only advertising block on the page.