The previous user is right - this application is brilliant if you use RSS feeds in a specific way, and I suspect it has been designed with that in mind.
If you have a large number of feeds, and read most of them every day, it is not helpful, as the "stream of news" makes it difficult to differentiate between them and the navigation is slow - click on item - pop out summary - close summary - click on next item ... in addition, you are showered with Growl notifications (one per feed), although those may be switched off in Preferences. The traditional three-pane reader is more usable in this instance.
However, if you have a small number of feeds, it is very slick.
I fall into the first category, but put a couple of my most-used feeds into Headline and like it!
The only real problem is that the pop-out item summary takes up a lot of screen space over and above the (small) main window, so that the space saving is not as great as it appears at first. There are also a couple of glitches; marking as read doesn't remove the item from view immediately, and I have found "refresh all" taking up a lot of CPU.
Overall, well-designed for a specific task.