Post your opinion here, I am very new to AdSense, but I believe link units can blend into your website alot, the disadvantage being that the user has to click twice.
it all depends on your page and ad/link unit location on the page; some times link units are attractive and compelling but they need 'two' clicks to get you paid!
I did a poll a few weeks back here on DP about Link Units, I was surprised that most people seemed to have decent success with them. On my own fledgling website, each page has 1 link unit and 1 large ad box. The link unit outperforms the ad unit, but not by a wide margin.