I used Papers2 and Papers3 for years until they eventually broke. I reviewed all the players 12 months ago and went with Paperpile. I think it’s wonderful. Now available for the iPhone and IPad as well as via Google Docs or Word on the Mac. Superb for collaborative writing via Google Docs - no need to share papers, every one adds their own citations. Easy to share papers with others also. Its cloud-based paradigm feels very future proof. One month free trial and easy to import from legacy systems.