M.MERRY  As a Uni student, this program is amazing, it actually works, looks great, is easy and is worth its price (they halve it if you prove your a student before you buy it) i highly recommend it and i'm constantly finding new tools and features!  
(Version 1.8.6)

BOMBADIL  I've been using Papers now for nearly a year and it is one of my top favourite and most useful apps I've ever bought.

I'm a geology Ph.D student and tried to use Sente but found it too biomedical-centric to be practical for me.

A few of the gems that I find especially useful:

- the ability to find & download articles directly within Papers

- the 'Authors' tab which searches the latest publications by a chosen author in your library - fantastic way to be up-to-date on a leading researcher's findings

- the export reference is easy to paste directly into the manuscript you're working on before you do the reformatting for the journal in question

Requests and suggestions for improvements:

- add some sort of highlighting functionality as in Preview app or Skim. While the Notes tab in Papers is useful, one tends to forget to look there if it's not selected. I find myself opening pdf's in Skim and making highlights there, then saving and returning to Papers.

- I love auto matching function to get metadata, but it only works for newer publications in my experience

(Google scholar integration is awesome I must say!)

Highly recommended app if you read a lot of articles, and it has inspired me to scan my paper reprints and go paperless :)  
(Version 1.7)

ALMAR  almost the greatest!  
(Version 1.9.3)

SIMONM  After using Papers, I don't know how I'd ever be able to write a research report again without it (or how I managed in the past). Papers was invaluable in helping me negotiate more than 500 papers I'd accumulated from the net during a literature search. The ability to search for key terms and quickly find references to them was a godsend.

I didn't use the online search tools, and it's a little frustrating that so few papers have quality metadata. Perhaps it's an area of improvement that Papers could intelligently determine paper title, journal and/or authors where applicable from the paper...? Given that this is much cheaper than Sente, I'd recommend it to any science student or professional researcher.  
(Version 1.9.1)

ALANTERRA  This is just a short review. I looked at Papers and decided to not use it.

Its user interface is well-thought-out, but it feels very incomplete. I could not figure out how to add a book or a book chapter as a reference type (and neither could a number of other people who posted to the forums). There isn't a manual that you can search for answers to simple questions, only some introductory movies plus forums on the developer's site. I ran into a number of refresh problems, which, until you figure out that they are just cosmetic bugs, cause pointless head scratching (do I not understand? is the program broken?).

As a way of browsing lots of pdfs, and for looking up bibliographic data in Google Scholar, this program is awesome. But if you move beyond its strengths, you find it hard to know how to continue.

I would definitely look at this program and see if it meets your needs, but also check out the alternatives, from Zotero to Sente (ie, from free to much more expensive).

A  
(Version 1.8.6)

THATO  So far this software has been great, the only other thing I would ask is a citeulike sinchronization option (or even a subscription like iCal calendars).

Since my demo is over, I might buy it when I get some money =(  
(Version 1.8)

GCOGHILL  I had to leave a comment just to note that this app has the best MacUpdate screenshot :)  
(Version 1.5.1)

ALMAR  Amazing program and works, looks great, easy to use.

I am a registred (and happy) user. I am wondering if in the next future the new release of Papers will be more integrated with Preview or have someof its basic functions (highlight text underline, link and more..).  
(Version 1.9.1a)

TOMECK  The approach of Papers is very interesting and the implementation of different online sources seems to be smoothly.

The scope, however, is science focused and I could not see a easy way to integrate social science online sources, in particular law journal etc. online sources. I have noticed the "plug-in" option but I haven't either the ability nor the time to do so.

It would be interesting to learn if Papers will be developed for social science in the near/middle future. If so, I would buy the software immediately only to support this project.  
(Version 1.7)

TIAGO  Not a big deal (at all), but the actual download is 20.2Mb and not 18.8Mb as shown on this page...  
(Version 1.7)