Mendeley

Mendeley got infamous for encrypting its local database, making switching to competing products more difficult. The claims of Elsevier, Mendeleys owner, in reaction to questions were perceived as "bizzare" and "Anti-Science"

[Edited by superluminar, January 17]

Although there are some bugs in Mendeley desktop and web import and also basic storage plan is very less, it provides all other features without bugging to take premium.

If you do a lot of archival research and/or citing of non-published sources (personal correspondence, manuscripts, etc.) in your work, I would not recommend Mendeley. It does not contain the fields you would need to properly cite that kind of material. Zotero is the best alternative I've found for that kind of reference/source management.

Since I believe that the comparison chart in em4020's review from August 2012 provides a slightly biased picture, I would like to introduce some more (slightly biased) comparison charts :-):

To those who didn't yet realize it: All these charts were created by the developers of the software tools being compared (also the one in em4020's review). I recently wrote about this problem in my Blog http://www.docear.org/2013/08/27/off-topic-which-one-is-the-best-reference-management-software-tool/

[Edited by joeran, October 30]

mendeley is really easy to use. and excellent retrieval of metadata. but: there is ONE big disadvantage. when you create comments in PDFs, mendeley does not export them. that means you cannot use PDFs and comments in other software. you are locked-in. source: http://www.docear.org/2013/10/14/what-makes-a-bad-reference-manager/ (i tried the export and it's true, comments in pdfs are exported in a crappy format).

i use zotero instead. not that easy to use but open source and excellent community. docear also looks nice.

A complete Comparison you find on http://www.mendeley.com/compare-mendeley/