Unison is generally considered to have the best reputation on the Mac, but it's shareware, and that turns some people off. Thunderbird is a free e-mail and newsgroup client which also works well. MT-NewsWatcher is another one to consider. Its codebase has been on the Mac in one form or another for many, many years, and as a result it's rock-solid. That said, it could use some UI work; much of its interface is rooted in OS9's older paradigms. It's not hard to pick up, but the aesthetics can be off-putting.
It's been a long time since I used Usenet for much, but back when I was more active I used MT-NewsWatcher. Unison came about since then, though, so I can't really say much about it.