Well the click through rate might be higher but in actual fact, it takes two clicks to earn something. That's why I don't use it at all.
I got very low CPC from link unit even though CTR is much higher. Sometime I got $0.01 per click! That's very disheartening!
I'd agree that ad units perform better than link units, but it's not either-or. The beauty of link units is that they are extra ads that run in addition to your regular ads, and take up practically no space at all. Since Adsense only displays 3 ad units per page, (plus 3 more link units), adding the link units gets more ads in a somewhat discreet way. For me, its either $0 w/o, or get a little bit extra with the link units.
I think, as always, the performance of these ad units will vary from site to site. There is no one magic formula for all sites. I'm not a fan of them because your users have to click twice for you to get paid
I like them, and I find if i click any ads on a site, I only click link ads. They just don't make a site look to cluttered.