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3rd will get a better click through than it ever previously did (good if you are in 3rd of course)at the expense of first and second places.
Google must be serps led (and I say that as a PPC specialist! so that's no small statement) for the quality of visitor experience as far too many adwords ads are still poorly targeted and too peripheral to the actual product/service searched for when compared to serps results.
Really hope they are just trialing for a day, as it's a bad bad move by google...needs removing like yesterday..
The more adwords intrudes into the serps rankings the more chance over time the quality of the user experience will go down.
Also having a list of 3 will mean more comparing than before I think..which means less conversion in the long run, not to a massive degree but already I'm seeing a drop of about 10% in CTR ratio's on some of my listings where I'm in second place, so if it had a CTR of 20% it's like 18% for yesterday.
I just think it's a bad move all round, it's not offering the best user experience in my view, it's too intrusive on the page.
If all adwords were ultra highly targeted and relevant then yes maybe but many are just too sloppy and not 100% relevant to the search that the user has conducted so over time I think it will show up as not good move
Google has been testing this on and off for the last 4 months and there have been so many different threads talking of this in this forum. So no surprises to most people here.
Also, there are more number of webmasters using AdSense then Advertisers using Adwords. Since this Webmasterworld, what do you expect?