Papers is available for the Mac, as well as the iPhone and iPad. Unlike social bibliographic apps such as Zotero and Mendeley, Papers lacks a web client. Instead, all your PDFs are held on your Mac, which can be wirelessly synced with Papers on the iPad or iPhone.
Papers makes extensive of use of the split view, in which the screen is split into two panes, the left-hand split becoming a popover when you hold the iPad in porrtait orientation. Almost all of the functionality of the iPhone version is crammed into the left-hand split. The popover displays the main interface categories (library, help, collections that you've put PDFs into), collections of documents, metadata for individual papers (which you can edit), as well as search results from a wide range of databases:
Some of these features you encounter as you drill down, say from library to list of papers, to details about a document, others you can access by clicking on the tab bar at the bottom.
Like the PLoS app, Papers displays PDFs. It doesn't use a page-turning effect, rather you swipe through the pages from left to right, with the current page indicated below in a page control (what Sencha Touch describe as a carousel control).
Given that the document being displayed is a PDF there is no interaction with the images or citations, but you can add highlights and annotations.
Papers is the first of the iPad apps I've discussed that isn't limited to a single publisher. If and article is online, or in your copy of Papers for the Mac, then you can view it in Papers for iPad. It is the app that I use on a day to day basis, although the PDF viewer can feel a little clunky. I think anyone designing an application reader should play with Papers for a while, if only to see the level of functionality that can be embedded in the basic iPad split view.