I have not much experience with link units. Is it good as an addition ? Should it be featured? Are you making more/less money on link units than the other ads? Many questions I am eager to hear your tought on this and your thoughts on link units in general.
are you asking about "Link Units" ?? Text units get more hits than image units, and proper placed link unit can get even more hits than text units.. I can't say which one gives you more $$, its google's strategy but there are some ideas that the link unit usually pays more
As you can have link units as well as your 3 ordinary ad units, it can only add to your revenue. I get a decent amount from my link units, and I wouldn't do without it. If you put them in the right place, and blend them well, then they will work nicely. Cryo.
It all depends on how you blend them. If you make them look like links containing useful info, they'll work very well.
I have a navigation menu on top of every page on my site. This menu consists of simple text links, with titles such as "Main page", "Articles", "Videos" etc. Right below this I have a link unit, and this makes the link unit look like a sub-menu, showing relevant text links to ads related to the content of my site. I find this gives a signicifanct extra income in my adsense earnings. Works good this way too. In addition I have a google search box below that which also has decent revenue.