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.