I blocked them all quite a while ago. I saw some ads that were pretty sleazy low quality, not what I expect from Google. When I looked through the list of ad networks they were mostly a bunch of nobodies that I would normally not deal with. So why is Google letting them in the back door? Google has pretty much always been the top tier in terms of quality.
Anyway, I haven't looked back and as far as my earnings are concerned, I haven't seen any impact whatsoever. My advice, dump 'em all and move on.