PUNKISH  Possibly the best of the breed.

Negative points: Not native Mac OS X (read, not Cocoa). Built with Qt, many controls are non-Mac like, and definitely are not polished. That said, it is not yet even version 1.

Positive points: Everything else. The program does a pretty decent job of sucking in PDFs and recognizing their meta-data. If no meta-data, it tries to recognize main components of a paper -- the title, author(s), year, etc. Has a built in PDF reader and annotator with highlights and notes. Can format citations many different ways (styles), has plug-ins for OpenOffice/NeoOffice. Where it really shines is that it also has a web version which syncs with the desktop. The web presence is fully social-networks buzzword compliant with friends, colleagues, networks, RSS, stats, etc.  
(Version 0.9.4)

PUNKISH  Pages '09 4.0.1 (build 746) has a major bug. Its entire Endnote integration is missing. I have verified this with Apple, filed a bug report, and they have "escalated" the report, whatever that means. If your life depends on Pages+Endnote integration, do not, DO NOT, upgrade to the latest iWork update.  
(Version 9.0.1)