I'm using qpopper. My sysadmin friends all tell me that's the premier pop3 server out there. And it's even available in an rpm. I'll leave a copy at http://steveo.syslang.net/qpopper-4.0.4-0.1.src.rpm
Depends on how it's used. If your users leave mail on the server, qpopper is horribly, horribly inefficient. Even UW-IMAP is better.
I can't really name any significant advantages to qpopper under any workload.
