If you use Adsense, make sure you check out the new report type that was released today. You can now view link units separately and get a lot more information. Previously you had to create channels to track link units, which was a pain - this new report is much easier and better. You can find it under "Choose Units" in the Advanced Reports tab.
If you don't use link units, now is the time to try. The thing I've found is that their value varies a lot depending on the site content - some of my link units earn a $15 ePM, while others fail dismally at around $0.03. It's especially important to experiment and tweak link unit settings for maximum effect. In some sites they just don't work and traditional ads are better. In other cases they can be worth many times what you'd get from traditional ads.