Thanks for your comments. Dr. Judson's article was interesting http://judson.blogs.nytimes.com/2008/12/16/defeating-bedlam/ and I'm sure many people have experienced similar frustrations.

We haven't done any head-to-head comparison (although we have some helpful comments that compare), but we have written posts on Zotero, Zotero Pt. 2, CiteULike, and Connotea and a brief post on EndNote and Refworks.