I’ve been using Papers for years. When Papers2 came out, I was quick (too quick) to jump in and start using it. It’s worst bugs got ironed out within a couple months, and I used it happily for a while. Papers2 would let you sync PDFs to your iPad for offline reading, but it was slow and a little clunky. Papers3 library syncing is not for offline reading and it is VERY slow and VERY clunky. And it relies on Dropbox for storage. The plus of this is that storage is free (as long as you have space in Dropbox); the downside is that they syncing isn’t clean and often fails.
Mendeley has proven itself the best at syncing your library and actual PDFs to the cloud (
you have to pre-download individual files for offline reading you can sync all PDFs in iOS in settings). Papers PDF viewer is still better, but it’s not worth the hassle: Mendeley syncs cleanly and the reader is fine. Not only that, but Mendeley has sharing options that make managing citations possible when writing a manuscript with co-authors (as long as they’ll use Mendeley).
Mendeley is also better than Papers at automatically finding the metadata for the paper (authors, title, abstract, etc.). The program simply works (most of the time), so I’ve given up and finally started using it. Almost exclusively.
PubChase syncs with Mendeley and recommends related papers weekly. (Update: the recommendations update daily, and they send out a weekly email with updates from that week.) They also have some pretty nice features, like a beautiful viewer for some journals and alerts when papers in your library are retracted.
Readcube still has the best recommendations. And they update daily, unlike PubChase’s weekly. And you can tell which recommendations you’ve marked as read, so it’s very quick to scan the list. But that’s really where Readcube’s benefits end. The enhanced PDF viewing feature is nice (it shows all the reference in the sidebar), but not really worth the slow-down in scrolling performance. The program is just clunky still. (I thought Adobe was slow!) And there’s no iOS/Android app yet. It’s on its way, allegedly, but I need it now! Readcube is really taking off, so maybe in a year it will be perfect. But not yet.
Edit: Readcube has a new version of their desktop application. Maybe it’s faster?
Wait, did the references sidebar disappear? No, wait, it’s there. Just not on by default.