I love Papers but they've been ridiculously slow with retina support; I wouldn't mention it, except it's the easy things that still haven't been done, such as coloured circles for tags, while more complicated things (many icons/artwork etc.) were done ages ago.
They released a major new version (Papers 3) just under a year ago. One of those 'fresh start' kinda releases; a re-think of what a reference app should be like, and substantially different from Papers 2. I still haven't upgraded because it doesn't do much more than Papers 2 did, but if you're coming to the product for the first time, it really is a great release. Highly recommended. Definitely the best Mac client out there, and they have a good iOS app too, along with solid Dropbox syncing. (Haven't tried their Windows version.)
Ah, they've moved since they started. Their new offices are indeed in London, though if you look at the copyright notice on the footer of that page you'll see it has a Netherlands address. Their careers page [1] does say their new offices are in London.
When they started, the two founders were both still mainly post-docs at the Netherlands Cancer Institute [2], and that's what I remembered, reinforced by the copyright notice I mentioned above.
They released a major new version (Papers 3) just under a year ago. One of those 'fresh start' kinda releases; a re-think of what a reference app should be like, and substantially different from Papers 2. I still haven't upgraded because it doesn't do much more than Papers 2 did, but if you're coming to the product for the first time, it really is a great release. Highly recommended. Definitely the best Mac client out there, and they have a good iOS app too, along with solid Dropbox syncing. (Haven't tried their Windows version.)