I had one of those Adsense "telephone consultations" back in October. In hindsight, I really think the Adsense people were doing a lot of consults because there was a big push at their end to get more image ads shown. (The holidays were coming up and I think they had a lot of inventory to move, just like a butcher pushes the chicken right before it expires.)
As a result of the consult I started running image ads. Now that I've been running them for a while I thought I'd mention a few things.
Income - Image ads didn't seem to make that much difference with my site's income. Clearly there was no big bonanza, but they didn't seem to hurt things either.
Targeting - One thing I did notice was that very few of the image ads were really on-target with my niche. I saw a lot of ads for department stores, coupon sites, Kodak, all of which are off-target for my site. (Although I guess these off-target ads were paying ok otherwise they wouldn't be shown.)
When text ads were shown they seemed right on target.
I saw some of those image ads that had that targeting where if you just searched for something (like a new car), a lot of the ads you see over the next while are all for that (like ads from car companies), which were all off-target (of course on purpose).
I don't mind the image ads. I assume I'll continue with them. The only thing I miss is that now that the ad topics are off-target so much of the time it seems more like advertising than before.
Before (of course who am I kidding) it was more like "here's a listing of products or services you really might be interested in or might help you" and now its just a "McDonald's" ad.