Papers appears to be an interesting application for those in the scientific community who need to read and manage a large number of papers as PDFs. It integrates with the online NIH database PubMed for searching and downloading. It allows you to organize articles not only by title but by author and journal. It even includes a full screen reading mode. There is nice review over at Infinite Loop by Jonathan Gitlin discussing how Papers has improved his own researching workflow.
The idea behind Papers, basically an iTunes or iPhoto for PDF journal articles, is a really good one, but I really wish it could be expanded in several areas. First of all, it clearly needs to support more online bibliographic databases and journal archives. As a humanist, for instance, I'd love a front end for JSTOR and the Philosophers Index (though perhaps I should not hold my breath since the developer calls Papers: "your personal library of science"). Secondly, and more importantly, I'd like to see Papers or a similar application offer a robust system for highlighting, comments, annotations, cross-linking etc. That's what I really need: a good tool to help me read articles (including and especially saving my notes), not just allow me to organize them.
In any case, if you need to manage professional journal articles Papers looks like a good start, though I did run into some bugs. It is presently available as a "Public Preview" and can be downloaded from mekentosj.com. It will eventually sell for €19 (~$25).