I have been a Papers user for many years and have recently decided to upgrade to Readcube. The functionality is very similar to that of Papers and everything works as expected and without issues. I was also able to migrate my Papers 3 library with no issues, which was a pleasant surprise. On the downside, the interface of Readcube lacks the slick design of Papers. The controls are very unfamiliar for an iOS app and might confuse some. The most important downside is the clunkiness of the PDF annotation experience, which is rather primitive compared to modern PDF annotation apps. I hope this will be improved in future updates.