As someone who spends pretty sizable money on search advertising ($1MM+ annual), I am a believer but I have noticed a serious drop in return on content advertising over the past few years and if fraud is occurring at a sizable level, it is within that network.
There isn't much fraud incentive within the actually search results save for competitor noise, however, via the content network, there is large incentive to inflate clicks.
As a result of testing and analytics, I've turned those off within my campaigns for the time being.
I attribute the drop in quality to the rise in business models of third parties around click revenue, whether blogs or 'search' engines which only return ads as results. Another factor of course is the search advertising is no longer new. At one point many users didn't understand that those were ads proper, that cat is now at of the bad so user behavior has adapted as well.