Two Excellent OSX Newsreaders for the average user are Unison (By Panic, the company that brought you Transmit and Audion), and Halime. Unison is reasonably priced Shareware, but is VERY user friendly and is continuing to grow better with each release; Halime is free, but is no longer being developed.
There are other more advanced Newsreaders such as Thoth and a few others, but they tend to be for the more advanced power users and aren't quite so user friendly in my experience. And they are all shareware.
