I had been a heavy user of reference management software"Mendeley".
Mendeley can watch folders in which reference PDF files are stored, and collect meta-data from the PDF files.
When I write a manuscript with references, I can pick up the necessary references from Mendeley in sophisticated ways depending on the editors (word and Latex).
Another useful function in Mendeley is the"group" function.
You can put a set of references which are cited in a manuscript you are preparing to a group.
You can export the entire bibtex of the references in the group to use in the latex manuscript,
and you can keep the reference set for future use.
I decided to pay $105 for two years to use more than five private groups in Mendeley.
Writing a manuscript without Mendeley was painful nowadays.
However, recently, Mendely crashes almost every time when I use it in my environment.
Because I could not fix it, I started using "Zotero" alternatively to Mendeley.
The transition from Mendeley to Zotero was not troublesome.
A short point of Zotero to me compared to mendeley is that there is no "watch folder" function in Zotero.
So if you want to newly save the references in the structured reference-storing folders and in Zotero, you have to save the references in the folder and you have to put the references in Zotero.
Because Mendeley could automatically collect the reference files from the watching folders, you don' t need to put the reference in Mendeley by yourself. However, In the case of Zotero, you have to do it by yourself.
This is a little bit bothersome.
The biggest advantage of Zotero over Mendeley is its stability.
Additionally,
the sub-collection function in Zotero, which is similar to the group function in Mendely, is free to use.
Tag function looks useful, though I haven't tried it yet.
pdf browser is system's default browser, which I found useful.
meta-data collection function is reliable.
The various citation styles (even JJAP!) are available.
The overall impression is that Zotero works quite well as an alternative to Mendeley.