Just so things are made clear, Mendeley is free. Any of the features what it currently provides for free, will remain free. There will be premium features coming up and those should have a specified cost. Also to keep in mind, Mendeley has sync features so you can sync your Zotero and CiteULike accounts with Mendeley. (Disclaimer: Beside being a long time user, I'm currently also community liaison with Mendeley)

It's not open source unlike zotero, and has plans to turn commercial. Which means all of a sudden you may be faced with the option of migrating to zotero or paying $$$. Most features are derivative of zotero, though collaboration with the wider research community is a lot better than zotero. Still I would recommend that instead of mendeley you use zotero in combination with academia.edu: for academics by academics, full functionality, no gimmicks.

I love this program. It is still a work in progress, but functional for the most part, and I've already written papers with it. The main points that made me move from EndNote and Zotero to Mendeley where:
-PDF MANAGEMENT. There is really nothing like this out there. Just drop a pdf file in the Mendeley window and it adds the paper to your database. That easy! You might have to correct a couple of tiny things, but for the most part it's done. Yes, in 1 second it adds a reference, renames your pdf file (AuthorYearTitle.pdf) and neatly stores it in the Mendeley folder on your hard drive! You can also build your library out of that black hole/folder with random pdf's you have. Just tell Mendeley to monitor it!
-WORD/OPEN OFFICE INTEGRATION: Yes, like your most valuable reference managers it functionally integrates with your word processor. I hope they extend support to LibreOffice soon! Also, they should allow users to modify styles since you can only use the ones already available (the same styles available for Zotero anyways).
-CLOUD STORAGE. It is cool to access your files anywhere, but you can not edit them from the web/IPad app so it's mostly useless to me unless I bring my laptop with me anyways. Also the cloud storage it's really expensive when compared with other companies (compare to Livedrive !), however, the first 500Mb are free so you can configure it to back up only the folders you are working with.
No doubt the best tool out there for academics and the such!

Mendeley is the only (free) app on linux that allows you to create comments in your PDFs, and does so with style.

mendeley is a lot more friendly than zotero, also don't depend of the firefox to work. Is already the best PDF/reference manager and gets better and better each version.

gordita_de_lisoform, that feature you mention that is missing in the new version is only temporarily gone. It is being redesigned and optimized and will return in one of the next releases. :-)

It has amazingly useful features like auto-extracting references from indexing sites like pubmed and google scholar... but it's a pity that the new version lacks the old feature that automatically extracted references from pdf articles stored in the hard drive :(

it works fine with me in ubuntu no crash no bugs and also it still beta so we should support these kind of projects
so glade to hear about this
thx so so much

Great in principle, in practice it's very very buggy, took me literally days to run to index my 3,500 pdf's (Ok, on a slow XP box Athlon 2100+) research library crashing all the way through, i'm persisting as there is nothing better out there especially for a PDF library manager and citation manager, maybe Endnote, Cathy, Scan, Zotero? So i hope later releases will fix the many many CTD and hanging bugs.

the sharing research papers section is a good idea but in my opinion papers is better in using google scholar or others scientific searc engine to identify the publication.

I'm glad you added Mendeley. I must say that it works pretty much the same on all platforms (Windows, Mac and Linux). It's a free desktop and web solution designed for managing and sharing research papers, discovering research data and collaborating online.