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While CTR is definately up a touch for me since they went live I'm not particularly happy with them on my red themed site where the ads used to blend nicely in.
I've also seen some site with colored background ads where the 'background images' are not transparent and actually become pictures on white backgrounds which make the ads looks completely ridiculous. I expected more from Adsense on this one I'm afraid - surely they could have done something that would blend more - or at least that would be based upon the ad design publishers already have - afterall they teach publishers to blend their ads....
Having spent so long blending and testing ads I think I made a mistake opting for themed ones, but other sites may well benefit from them.
I may run themed ones on certain sites to test them (my visditors may love them for all I know).
I suspect there may be a connection with cache and business keywords, as pages with any kind of business topic don't display the themes (though, the home page is heavily business oriented).
Also, click through rate is down by about 1/3rd of where it normally is at this time of day. It's difficult to say whether this is due to the themes, though, as it could be a sudden decline before Christmas. Last year CTR dipped around the 21st/22nd through to after Christmas. On the other hand, the random appearance of themes could be putting visitors off.
I'm disabling themes, for the time being at least, until I understand better what is being shown to the visitors (especially those in other countries). Perhaps we need a theme preview tool that can be geotargetted.
Thing is Google HAVE tested themed ads and must surely think they have merit to roll this out.
So on second thoughts, I will run them on "suitable" sites only and see how things go.
It is difficult to test at this time of year though.
** edited to add:
I take that back... I see them on the other site I mentioned in my previous post (leaderboard on dark background) and now I don't see the transparency, but they still look bad!
I think it takes like 24 hrs. for them to be removed after you disable it. I'm still seeing them now. For some reason, the CTR didn't change (just lucky?) on my stats.