I do very well with link units. They occupy such a small amount of space, that I can usually embed them very tightly with my menu system. The CPC for link units tends to be smaller, but often a user will click thru a half dozen of them at a time. The best part is that link units don't count towards you maximum three ad units, allowing you a fourth.
I've also had relatative success with link units. They are very unobtrusive, so it's much better having them than not. In any case they are sure to get you at least a few extra dollars a month... little revenue is better than none. Best bet would be to try it for yourself and see how they perform on your site.
In my case, my ctr for link unit is a lot higher than my ad units due to the fact that I only have it displayed when I dont have my own data to display. When they get displayed, they tend to get click thrus. CPC tends to be slightly higher too.
In my case, from three websites running link units, on two of them I'm getting more revenue. Since the first links are so appealing and related to my users I tend to get quite a nice amount of money from them!
I was skeptical about the link units but I was quickly converted once I put them on one of my sites. They have performed very well for me.