My theory is that certain products are more visible and less conceptual (and vice versa). But obviously I have yet to put this into practice..
Also, it may increase your CTR but I have yet to really see a way that it might decrease your CPA.
One problem I have is for an image ad to run, a text ad must also be in the AdGroup. I have a product that I believe will have much better conversion if people see the product before they click. That being the case, I am willing to (as least until I know better) pay more for a image ad click than for a text ad click. As long as I must have a text ad in the group, I can't raise my bid without pushing the clicks for the text ad into negative ROI.
That being said, I won't raise my bid. Everyone loses. This is a major problem.
Adwords used to include "Image Ads" as a selection in their reports but that's gone now. I guess it's not working out the way they had hoped. Or maybe it will just take time for more people to jump on-board.
I've had some ads show up in the 468x60 format (only 2 ads), but I haven't seen any other sizes show up yet. I've actually paused the one ad set that had all 4 ad sizes, though.
CTR results seem to be better than normal AdSense, sometimes almost as good as my AdWords entries (which are, admittedly, doing extremely well positionwise). I think, all in all, this will be a good idea as long as ads are targeted properly.