So a site I manage that is over 200 million page views a month was affected by the recent text ad format change by AdSense. We are one of the publishers no longer serving the grey arrow versions. They worked incredibly for us, but that's over..time to move on.
Now looking at the performance data, I have made our campaign all rich media/image ads...no more text ads. Before, we were very happy and ran text ads almost exclusively. Now I am thinking if I am going exclusively rich media/image ads, perhaps it is time to consider another ad network than AdSense. Given such high traffic our site is, AdSense seemed liked the best option, but now that texts are out for us perhaps there are better options/approaches.
You won't know until you try it of course so I'd run at least a week long test. I'd also think you'd have a constant stream of emails from competitive ad networks seeking your business on a site with that many page views so finding an alternative to AdSense shouldn't be to difficult if you decide to go that route.
My image ads killed it last year but aren't earning as much this year (although the CTR is about twice what it is for the text ads) I've reduced their coverage a bit on the site, and will test again when my traffic picks up more.
(Instead of running text/image ad units, I rotate one text only and one image only, so I can keep closer track of which is performing better)
Images ads always seem to have a lower CTR than text.
I find the opposite. Almost quadruple the click-through-rate for images over text ads. So this is probably something that will be variable for any given site.
The only issue we've found with image-only, is that there is a much smaller pool of adverts. So our users get shown the same ads over, and over, and over. So EVENTUALLY CTRs on regular visitor sections of the site drop. Also, there are far more occasions of white space instead of an ad when choosing image-only.