This is definitely a useful add-on!
The possibility to restrict certain sites to specific containers is really a plus as compared to the basic containers that come with Firefox. It gives less risk to open a site in a "wrong" container or the default one.
It's also quite cool to know it impairs giant sites (Facebook, Google and others) to track one's online activities. I created a google container by the way.
However, the lack of sync ability for the moment costs one star, though it is foreseen by the developer. I found a temporary workaround that is to sync containers.json and ../browser-extension-data/@testpilot-containers/storage.js (both found in one's Firefox profile) through a sync application (FreeFileSync does it very well, download it from here: https://www.freefilesync.org/)