Within my firm we often get captchas from google, because we have about 50000 staff connecting through our Germany gateway, and to Google that looks a bit like some sort of DoS attack or attempt to drive up clicks.

Never seen them as a problem - if I get one I can't figure out, I click for the next one.

Also not a fan of AdSense though - but I am happy that that is the payment model google use as it means I can avoid most of its effects.