I did a poll and nearly 30% of the people who voted said they used Sync! It's stayed around that percent the whole time too!
EDIT: Here is the poll I forgot to put this here like five times lol https://strawpoll.com/wby5A21R1yA
App seems nice and I have no complaints with it but it's not FOSS and it has ads. It would be cool if there were easy ways to make revenue off of FOSS that dosen't rely on donations so I understand why it would be proprietary.