I have used most of the major reference management platforms at one time or another: EndNote, RefMan, Zotero, and Mendeley. Most of my time in grad school, I used what my advisor used to make collaborating easier: EndNote at UCF and RefMan at Arizona. During my Ph.D. I played around with Zotero but found it clunky. Back in those days, Zotero only worked with Firefox, and it didn’t sync files, so I had to have a separate way of organizing my PDFs.
I was an early adopter of Mendeley, and I loved it from the start. It integrated citation management with PDF organization, synced across platforms, and had an interface that was about as intuitive as you could ask for. Importing PDF metadata didn’t always work, but that was fine with me. It was also free, for a limited (but pretty generous) amount of storage. It became such a vital part of my workflow that, when I had filled up the free storage, I was happy to pay for more. I’ve been paying Mendeley $54.99/year since 2011.