I work in history and psychology, and since 1992 I have used Endnote, Bookends, Sente, Papers, Zotero, Mendeley, Bibdesk, and probably a few others I can’t remember. The ones I used for an extended period of time were Endnote, Bookends, Sente and Papers.
I thought I had settled on Papers, but the transition from version 2 to version 3 was a disaster. If you do a bit of research you will find old posts about people losing data, the app not working properly for quite a long period of time, and inadequate technical support.
I cut my losses and returned to Bookends, which was the program I had used most in the past. I liked Sente in many ways, but it did have trouble formatting bibliographies at times. Bookends is not a pretty program, but it is VERY powerful. And it just keeps relentlessly working. The developer is very good at supporting the program, though terse in his responses. Bookends integrates very well with Tinderbox and DEVONthink, not to mention Scrivener. The are all part of the “artisanal” family of programs with regular discounts, and seem to support each other.
I have 2,217 references in my collection, 866 with attachments (which can be any sort of file, or even a folder with multiple files in it – I think I have about twelve attachments for one of my references) and all of this syncs to my phone or iPad (there is an iOS app). Having my entire bibliographic database on my iPhone is nice, though I don’t use the facility much.
In short, other programs come and go, they break, or fade away, but Bookends just carries on. It has its quirks, like any software, but it works. Solidly. Oh – and the User Guide is a 445 page pdf. The documentation is very comprehensive.
There is a forum, too:
You might like Papers more, but I wouldn’t touch it after the catastrophe of the version 2 to 3 transition. Mendeley and Zotero also have their supporters, but I don’t have any recent experience of using them. I just found them a bit more inconvenient when I tried them.