I predominantly use and have expertise in EndNote as ACU subscribes to this. Once a user sets up and learns the basics of EndNote, I can assist them also in building their skills to tailor EndNote to meet their research needs. It is an excellent program, but if you want to collaborate, not so easy, unless you use EndNote Online. The desktop version, however, is more powerful and I prefer this version (as long as you back up a compressed version of your library to a USB or the cloud like Dropbox).
RefWorks I have yet to grasp. I have issues installing the Write'n'Cite plugin and this was also difficult when assisting a student on his Mac laptop. It was frustrating. I am much more competent with EndNote.
I have experimented with both Zotero and Mendeley. I have the Zotero browser extension in my Firefox browser, but I haven't used it enough to know the benefits. Also, I had completed the first two parts of a three part Mendeley Librarian Certification program offered by Elsevier. The beauty of Mendeley is the collaborative aspect, where you can create a private group and share papers, and work on a project (like a publication) with others. There are also public groups where you can share citations, and I like that social networking aspect of Mendeley. I had two academics from ACU ask me if there are any tools to use for sharing references and creating a paper with a peer from another university as they worked on a publication. At the time I only knew of EndNote and it wasn't the best for collaboration. If only I knew more about Mendeley to provide information to these researchers back then so a more informative decision could be made.
Each reference management tool has its strengths and weaknesses. It is a matter of assessing your needs before you start on your project (eg. is it individual or collaborative) and making a call on which tool is most appropriate to use from the start.