I've recently tried a click-heatmap and noticed that the clicks drop significantly below the ads. I thought maybe visitors click the ad and don't come back.
Today I've received an email requesting a listing, which was already on that page, just below the ad. Later, he confirmed that he didn't scroll past the adsense ad and didn't see it.
I'm using 336x280 ad units. I think 300x250 wouldn't make much difference. And smaller units feel too narrow.