Another advantage with link units is that they can be accommodated in the limited space available in your site and if you try to blend them with links in your content, then your ctr will go up...
If you have category options or sub pages link in sub header or navbar than you must try link units below them . May be by mistake but we get good clicks on such ads . I am using them from last 2 years and they provide good income but you need to find best place to point them .
Link units have a high CTR because of the fact that the keywords on Link units is highly relevant to the content of wed page that is why people use link units even though they pay slightly lower that text and image ads.
I still get better results with medium rectangles front and center above the content below the title of any given post.
It look like normal link and many people will click on it. Or saying that link unit has higher CTR generally
link units have low eCPM. i believe skyscrapper performs better in terms of CTR and eCPM (atleast it works that way for me)
in my experience link units don't pay same as other ads. I experimented with one under menu, it got me around lets say X$ and hundred clicks month. I replaced them with normal ad units and CTR droped by 1/5 but total profit increased to 5X. It's a big difference and CTR doesn't matter if ur getting paid cents!