I used Dropbox from 2009 through last month or so. Switched over to Syncthing. I was a little bit worried about stability but soon after I sampled it I felt convinced that it was the right move. I was previously using Dropbox as a kind of makeshift backup as well as a sync tool, but now I have a desktop again and run automated backups on that using Backblaze, so I'm a little bit more in control over the data by doing my own syncing.