Wouldn't have guessed this -- since link units require an extra click.

Quoted: Recently, Julie experimented with optimizing her link unit ad placement. She removed the existing 120x90 link unit from the uppermost right corner of her pages, and instead placed a 468x15 link unit at the top of her articles. In doing so, Julie found a convenient, unobtrusive location and saw her readers naturally gravitating towards the link unit. Her efforts led to an immediate doubling of her revenue -- an increase that has been sustained since the move.