Personally, I am writing my Ph.D. Thesis in Scrivener and managing the bibliography in Tinderbox. In the Github repository linked below, you can check out how I have things set up, should you feel inclined to do so.
Having said that, here is my take on reference managers.
In case you plan on converting your work using Pandoc, I suggest using Bibdesk. It is plain and simple to use, fast, free, open-source, and Applescriptable. Also, you’ll be working the whole time directly with the BibTeX file. In case something is not at the right place and you can’t or won’t fix it using the app, just open the file and make the change right there. Nothing beats this, IMO.
Zotero doesn’t cut it for me because it doesn’t let me choose and input the cite key directly; for that, we need a plug-in which makes it very slow (Better Bibtex, or something akin). (It does now, however, have an interesting pdf reading environment in which it is very easy to interconnect notes, but that is another matter).
I have used JabRef several times and found it good for specific purposes, but, in general, it is very cluttered.
Bookends is an overkill if you don’t need integration with other apps such as Mellel or Word. I find it too loaded with features that I don’t need and somewhat slow for browsing. I keep it installed but don’t use it very often. (Also, updating the app is a pain. Every single time you have to go over to the website, find the link, download, and so on).
Endnote is very expensive and I personally can’t find any reasons to justify the cost. I also find that most of the other options are easier to use.