Earlier quoted context omitted.
Mendeley was originally a startup. Precisely why people even started embracing it (though I didn't, something always felt fishy for me) and then they got acquired by Elseiver. I have nothing but good things to say about about Papers for Mac though.
I've never used Papers, but have had colleagues that swear by it. However, it is not open source, so you still run the risk of the developers selling out just like Mendeley.
The nice thing about Papers is that you download every pdf and have a local copy. Of course the developers could change how it's done.