KLM used to have some on the trays of some of the F100. When no paid ads were in place, the ad agencies blurb was on show. it stated something like a bus advert is seen for 5 seconds, a newspaper 10 secs etc, but this seat tray ad is seen for x hours. After looking at the same add for that amount of time, my thought was I'd be b*ggered if I'm going to buy you product.
I don't think the finances really add up to make it worthwhile. In a days work, a bus ad will be seen by thousands of people, but in an aircraft, assuming all trays with the same advert, for 8 sectors in a 737, there would only be 1184 people looking at it.
Back to the F100, and on one flight some bright spark had replaced a row of ads with those postcards you get from the phonebox. That would be worth looking at for an hour!