I'm using it, it seems to work fine, and works/syncs on mobile as well (mobile Firefox that is, not for other apps AFAIK), and on my work computer's Firefox instance. I seem to recall attempting to change my Firefox sync password once and it notified my that I would need to export and re-import the passwords (IIRC), which I took to mean they are careful to never get full access to them unencrypted on their side. While less convenient, I like how that's set up if it's how I think it is.

For every device I have logged in with Firefox sync it just works as you would expect.