Here is a Berkeley site with nice tutorials, in depth descriptions of each program, FAQs and more. Discussed are EndNote, RefWorks, QUOSA, and Reference Manager.
http://www.lib.berkeley.edu/PUBL/endnote.html
There is also this detailed side by side comparison of several programs including RefWorks, Biblioscape, Bookends, Library Master, ProCite, EndNote, Reference Manager, and Papyrus:
http://www.burioni.it/forum/ors-bfs/text/6e12400-42.html
Tufts also has a nice page comparing Endnote, RefWorks, and Reference Manager.
http://www.library.tufts.edu/hsl/education/endnote_refworks.html
http://healthlinks.washington.edu/howto/citation_tools_comparison.pdf