A while back, I tried out Papers as a reference manager. I liked it, but since I'm regularly using three different computers (and the license allows for two) I decided against using it and settled for BibDesk that's also pretty good. Especially since I'm mainly using LaTeX for my writing.
Since then, though, I've moved to writing more and more in Word since that is what my biology colleagues are using, and there I use EndNote, but as a reference manager I don't really like it much. I need it for work, but for managing my references I'm just not that attracted to it.
Now I am finding myself in the situation that I need to share a reference list with some people I'm working on a review paper with. I have no idea how to share such a list using Papers, and while it is possible with a combination of BibDesk and Dropbox, it isn't quite as easy as I would like and I would need to to export to EndNote from time to time, so I looked around to find an alternative.
Synchronizing the references between my computers is really easy since the reference list is on a web site, and sharing a reference list is just as easy since this is built in.
The desktop application is pretty easy to use
but I'm finding that I like the web interface more (but I would love to have auto-completion on keywords there as I have in the desktop application).
The main feature I missed when I tried out Mendeley the first time was a way to search for publications within the tool. As far as I can see that is still missing, but if I'm using it through the web interface it isn't such a pain. I can search in PubMed or Google Scholar or such, Mendeley can import from search results.
I haven't tried this yet, so I don't know how well it works, but Mendeley can also synchronize with a BibTeX file which makes it perfect for my typical use. It can also export to EndNote (but there's more manual work involved here; not so much an issue for me since I don't need this quite as often as exporting to BibTeX). It also exports directly to Word, as far as I can see (but I haven't tried that yet), so perhaps I can get my colleagues to use it instead of EndNote to avoid the problem completely. We'll see.