A question I ask myself is how much do I make on that Nth ad unit. Is it worth the user frustration? Some blog units have so many ad units (Chris Pirillo) that after a minute the window continues to shift as new sections of the screen get incorporated. How much money did Chris make with that 10th ad unit that caused the textarea to disappear off my mobile screen. Thus frustrating me and causing me to write this instead of reading his blog and leaving a comment. I hardly doubt it was worth it to him. On the other hand, he's likely richer than I from this same strategy.