I use Bookends since version 7. It is my main bibliography management software. I switched from Endnote, and never looked back (though newer versions of Endnote are said to be better).
Bookends never crashed on me, and always worked as advertised. It has very flexible options for formatting your references, and a lot of built-in formats. You can also create your own, of course. The built-in PubMed search tool works very well and saves a lot of time, too.
Of the negatives, it has nothing like EndNote's cite while you write. This means that your Word files will be littered by full-text reference placeholders in curly braces, like, say, "{Goldman 2003}". When you are done writing, you will tell Bookends to "scan" the document, which is when the references will be converted to their final form. This mode of operation can be a minor inconvenience if you are trying to format a paper so that it fits within journal's guidelines. However, Bookends is way more stable than Endnote has ever been, and new versions of Word are usually supported right away. With Endnote, you will wait for the next version, sometimes for a few months.
I also own Papers. It is not really comparable to Bookends. Papers shines for reading papers and for doing lit search. There are plugins for most every academic search engine, with a notable (and expected) exception of SciFinder. Papers is not nearly as nice as Bookends for formatting bibliographies. On the other hand, I would not use Bookends for reading and collecting PDFs. That would just be painful.
Papers and Bookends integrate nicely when you have both installed. You can export references from Papers to Bookends.
I have no experience with Sente, unfortunately. But I love Bookends.