While Mendeley (particularly the desktop application) is quite good as a reference management system, it does have its limitations. You should be aware of the following shortcomings:
Citation Import Problems: Mendeley Web currently will NOT import citation files from the following databases (these do work fine with Mendeley Desktop, however).
Mendeley Importer Extension (Google Chrome) - This Google Chrome extension interferes with ProQuest eBook Central, causing eBook pages to load and reload continuously, making interaction with the book difficult if not impossible.
Range of document types: Mendeley has a relatively limited range of document types, compared to other reference management softwares (EndNote, RefWorks, Zotero).